When I transfer money to my Roth IRA account, it keeps it in a cash account until I invest it (doesn't do it automatically). It takes me around 6 months to get to my $5500 limit. Is it better to invest once after I've reached the limit or invest every time I transfer money?
This depends on the terms and conditions of your IRA account, and those of the investments you have chosen.
In general, you are better off investing as quickly as is feasible given those terms. Money in your cash account doesn't earn much of a return, so the quicker you get money into something earning a return, the better.
However, pay attention to the fees and costs associated with investing. If there is a per-transaction fee, you may want to consolidate, as it may be more efficient to do so - after all, if you contribute $500 at a shot, and it costs you $5 to make a trade, you're paying 1% off the top to make that trade if you make 11 of them, versus 0.1% to make 1, so the question is do you earn that 1% back over the course of the six months? That will depend on what you are investing in.
More than likely you're going to earn more than 1% over the course of the six months, so it's probably worth investing it in pieces still in that situation, but if the transaction cost is higher, or the time differential lower, you may have a less clear-cut answer. I invest at Vanguard in their funds and have no transaction fees, so I have a more obvious answer (invest as soon as possible).
You also need to consider whether you have minimums to pay attention to - maybe your investment is something you can only buy whole shares of, for example, or you might have a much higher fee if you make small transactions. In that case, you should wait until you have the minimum to make that transaction if the fee is more than the return you'll get.
So the answer is - make the transactions as early as you can, subject to considering the fees you will pay for making them.
If you are like most people, your timing is kind of awful. What I mean by most, is all. Psychologically we have strong tendencies to buy when the market is high and avoid buying when it is low. One of the easiest to implement strategies to avoid this is Dollar Cost Averaging. In most cases you are far better off making small investments regularly.
Having said that, you may need to "save" a bit in order to make subsequent investments because of minimums.
For me there is also a positive psychological effect of putting money to work sooner and more often. I find it enjoyable to purchase shares of a mutual fund or stock and the days that I do so are a bit better than the others.
An added benefit to doing regular investing is to have them be automated. Many wealthy people describe this as a key to success as they can focused on the business of earning money in their chosen profession as opposed to investing money they have already earned. Additionally the author of I will Teach You to be Rich cites this as a easy, free, and key step in building wealth.