I agree with all the people cautioning against working for free, but I'll also have a go at answering the question:
When do I see money related to that 5%? Is it only when they get
bought, or is there some sort of quarterly payout of profits?
It's up to the shareholders of the company whether and when it pays dividends. A new startup will typically have a small number of people, perhaps 1-3, who between them control any shareholder vote (the founder(s) and an investor). If they're offering you 5%, chances are they've made sure your vote will not matter, but some companies (an equity partnership springs to mind) might be structured such that control is genuinely distributed. You would want to check what the particular situation is in this company. Assuming the founders/main investors have control, those people (or that person) will decide whether to pay dividends, so you can ask them their plans to realise money from the company.
It is very rare for startups to pay any dividends. This is firstly because they're rarely profitable, but even when they are profitable the whole point of a startup is to grow, so there are plenty of things to spend cash on other than payouts to shareholders. Paying anything out to shareholders is the opposite of receiving investment.
So unless you're in the very unusual position of a startup that will quickly make so much money that it doesn't need investment, and is planning to pay out to shareholders rather than spend on growth, then no, it will not pay out.
One way for a shareholder to exit is to be bought out by other shareholders. For example if they want to get rid of you then they might make you an offer for your 5%. This can be any amount they think you'll take, given the situation at the time. If you don't take it, there may be things they can do in future to reduce its value to you (see below). If you do take it then your 5% would pay you once, when you leave.
If the company succeeds, commonly it will be wholly or partly sold (either privately or by IPO). At this point, if it's wholly sold then the soon-to-be-ex-shareholders at the time will receive the proceeds of the sale. If it's partly sold then as with an investment round it's up for negotiation what happens. For example I believe the cash from an IPO of X% of the company could be taken into the company, leaving the shareholders with no immediate direct payout but (100-X)% of shares in their names that they're more-or-less free to sell, or retain and receive future dividends. Alternatively, if the company settles down as a small private business that's no longer in startup mode, it might start paying out without a sale. If the company fails, as most startups do, it will never pay anything.
It's very important to remember that it's the shareholders at the time who receive money in proportion to their holding (or as defined by the company articles, if there are different classes of share). Just because you have 5% now doesn't mean you'll have 5% by that time, because any new investment into the company in the mean time will "dilute" your shareholding. It works like this:
- Initial investment into the company: $1 million
- Of which equity given to you: 5%
- New investment coming into the company: $9 million
- New total shareholder capital in the company: $10 million
- Of which your capital is: $50k (5% of the original $1 million)
- Your new shareholding: 0.5%
- Money paid to you so far: $0
Note that I've assumed for simplicity that the new investment comes in at equal value to the old investment. This isn't necessarily the case, it can be more or less according to the terms of the new investment voted for by the shareholders, so the first line really is "nominal value", not necessarily the actual cash the founders put in.
Therefore, you should not think of your 5% as 5% of what you imagine a company like yours might eventually exit for. At best, think of it as 5% of what a company like yours might exit for, if it receives no further investment whatsoever.
Ah, but won't the founders also have their holdings diluted and lose control of the company, so they wouldn't do that? Well, not necessarily. Look carefully at whether you're being offered the same class of shares as the founders. If not consider whether they can dilute your shares without diluting their own. Look also at whether a new investor could use the founders' executive positions to give them new equity in the same way they gave you old equity, without giving you any new equity. Look at whether the founders will themselves participate in future investment rounds using sacks of cash that they own from other ventures, when you can't afford to keep up. Look at whether new investors will receive a priority class of share that's guaranteed at exit to pay out a certain multiple of the money invested before the older, inferior classes of shares receive anything (VCs like to do this, at least in the UK). Look at any other tricks they can legally pull: even if the founders aren't inclined to be tricky, they may eventually be forced to consider pulling them by a future new investor. And when I say "look", I mean get your lawyer to look.
If your shareholding survives until exit, then it will pay out at exit. But repeated dilutions and investors with priority classes of shares could mean that your holding doesn't survive to exit even if the company does. Your 5% could turn into a nominal holding that hasn't really "survived", that entitles you to 0.5% of any sale value over $100 million. Then if the company sells for $50 million you get $0, while other investors are getting a good return.
All of this is why you should not work for equity unless you can afford to work for free. And even then you need to lawyer up, now and during any future investment, so your lawyer can explain to you what your investment actually is, which almost certainly is different from what it looks like at a casual uninformed glance.