C-Mac, I practice about twice a month, and it's usually on a Saturday or Sunday rent-a-lane when there's little to no oil and some carrydown.  Practice really depends on your expectations.  If you want to score, then off hours practice is tough.  I try to work on execution -- timing, ball speed, release, square at the line, follow through, etc. using the following strategy.
 
Game one, straight up 5.
Game two, swing 10 to 5.
Game three, straight up 10.
Game four, swing 15 to 10.
Game five, swing 15 to 5.
Game six, swing 20 to 10.
Game seven swing 25 to 15.
Game eight if I have time, I shoot 10 pins on the first ball and 7 pins on the 2nd.

I use my tamest urethane ball and concentrate on hitting the intended mark at the arrows.  A lot of shots don't make it to the breakpoint, but if I hit my mark and it's heading in the right direction, I'm OK with that.  As you can imagine, this yields a lot of low scores because you're not really playing the lane.  But I've found it helped with my accuracy when I have to move away from my A game.  And to be honest with you, I'm not even sure what that is any more.

Thanks for reading, and good luck.
Joe

C-Mac what I do is get to the lanes early enough to get a game or two warm up after they oil the lanes for league. It usually takes about an hour to oil the lanes, so you should have enough time for 2 or 3 games on the league shot, that is if they allow practice after they oil the lanes, some don't. If they don't try and bowl after the first shift on a pair that was not used, or a pair that had a few absentees on it, that is if they don't have a second shift.

Twice a month?  No practice at all?  Practice every day of the week?

It depends.  What are your bowling goals?  Do you just want to improve this year over last year?  Do you want to shoot an honor game?  Do you want to shoot 800?  Do you want to be competitive in local tournaments?  Do you want to earn your PBA card?  Your goals will determine how much you practice.

My personal goal is to earn my PBA card and bowl in PBA Regional tournaments.  When I practice I keep a few things in mind:  

1 - accuracy is more important than striking on a burnt out condition.
2 - spares are more important than strikes.  I can get strikes, but I will have to shoot spares eventually.  I was at the South Regional tournament yesterday and one pro, Owen, told me how he lost nearly 100 pins in count for a few missed spares.  That 100 pins would have put him in match play.
3 - regional events start with 8 games of qualifying.  If you can't bowl more than 4 or 5 games without getting tired, then you have a problem.  I practice 2-3 days a week and bowl 10-12 games at a time.  I grab a lane on Wednesdays when I can get a pair for 6 buck for 2 hours.  Games then end up costing me about 50 cents apiece.  I also go on Sundays when games are a buck apiece.

My week in a nutshell:
Sunday - practice at about 12:00 when the center opens.  I can get the lane man to put down a fresh league/tournament/sport shot for me.  10-15 games.
Monday - work, then league at 7:30.
Tuesday - work, time for my family.
Wednesday - work, practice 2 hr block.  Usually 12 games.
Thursday - work, league at 7:30
Friday - work, local tournament for a little cash
Saturday, work @ bowling center

That's about 31-36 games a week, give or take.  Based on my goals, that's what I do.

What you do is determined by what you want out of bowling.
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Nut

If you wonder if you're good enough, then you're not good enough.

Oh, I disagree.

There is form practice, sparemaking practice, equipment practice(balls, shoes, wrist braces), scoring practice(I love no taps for this), and match practice.

All elements can assist in competition.

REgards,

Luckylefty
PS a friend of mine recommends, a three game practice set of
1. corner pins, 2. strike balls, 3. One game for score.  I don't know he had an 845 in his league finals last year and then had a 300 in the rolloff.  Sometimes it works!