General Brewing

I am new to this site and making wine, but thinking about adding beer to my hobby....I don't drink that much beer but enjoy it....If I made a 5 gallon batch of beer and placed it in a keg, with CO2 and cooler, how long before it will go flat???

Beer will remain carbonated as long as there is CO2 in the headspace equal to the carbonation of the beer. For most beers, this is 12 to 15 lbs.

I don't keg, so I can't help you with that.  I can tell you though, that you'll have no trouble getting it to disappear.  Your friends and relatives will always want to sample it.  Join your local homebrew club. 

When you host a party, instead of filling your cooler with Bud, Miller, etc., fill it with your homebrew.  If you have a variety of beer styles on hand, most people will find something that they like.  I even take a case to parties that my friends are throwing.  It's always appreciated.

You can always take a keg to a party and share it.

If you think it's going to take a while to empty the keg, the bigger picture is picking a style that benefits from time rather than something that is better when it's young.
Complex beers do well with time.
Something like a hefe is better young.
Do you like stouts or porters?

If you're not drinking a lot of beer, consider brewing smaller batches. 2.5 gallons will be perfectly happy sitting in a 5-gallon keg under CO2. That would be especially appropriate for time-sensitive beers, such as Bavarian weizens (yum!).

If you haven't done so already, read John Palmer's "How to Brew". An old version is available free on-line at    howtobrew.com

Are you aware of the concept of a, "slippery slope"?