I wouldn't say you should have any particular limit, but it can't hurt to have a higher limit. I'd always accept the increase when offered, and feel free to request it sometimes, just make sure you find out if it will be a hard or soft inquiry, and pass on the hard inquires. From my own experience, there doesn't seem to be any rhyme or reason to the increases. I believe each bank acts differently based on the customer's credit, income, and even the bank's personal quotas or goals for that period. Here is some anecdotal evidence of this:
I got my first credit card when I was 18 years old and a freshman in college. It had a limit of $500 at the time. I never asked for a credit line increase, but always accepted when offered one, and sometimes they didn't even ask, and in the last 20 years it worked it's way up to $25K. Another card with the same bank went from $5K to $15K in about 10 years. About 6 years ago I added two cards, one with a $5K limit and one with a $3K limit. I didn't ask for increases on those either, and today the 5K is up to $22K, and the 3K is still at $3K.
An even larger disparity exists on the business side. Years ago I had two business credit cards with different banks. At one point in time both were maxed out for about 6 months and only minimums were being paid. Bank 1 started lowering my credit limit as I started to pay off the card, eventually prompting me to cancel the card when it was paid in full. At the same time Bank 2 kept raising my limit to give me more breathing room in case I needed it. Obviously Bank 1 didn't want my business, and Bank 2 did. Less than a year later both cards were paid off in full, and you can guess which bank I chose to do all of my business with after that.