There are a few sites that you can hire from, but to pay a monthly rate at the mo with only a handyful of dire launch titles out at the mo for the ps3 it would seem pointless at the moment.
I don't like hiring 360 games as most of the disks are scratched to high hell and to get the most out of a game you need to play it enough to be able to play it well online/live, so you might as well buy it.
I tend to get a good general review of a game from as many sources as poss before I tend to pay out full whack for game, but I generally get the ones I know I will play and there are a lot of 360 games that are less than £20 now.
I used to hire dvds from Blockbuster, but it got to a point where I couldn't find a single title I wanted to hire and ended up not getting my money's worth, and all the new titles were always delayed or I had to wait in turn, I cancelled that as it ended up being a waste of money.
So an answer to your questions in my opinion is
1 Check
www.gamerentalguide.co.uk and check reviews etc
2 Depends on the person, PS3 games are going to be full priced for a while yet, 360 are already dropping/budget ranged.
3 As long as you want really but the long you have the 1 title then the less choices you get out of your monthly subscription.
Best bet for ps3 games, buy the games you want, then flog them on ebay when you're done with them, as for a while they will hold their price well.