How well will a keg hold pressure?

My current set up is a dual keg CO2 system and a single nitro system. I've got a nice brown ale on one of my CO2's and lovely stout on the nitro. While I am quite ready to brew a third beer, my pregnant wife has asked for me to make some root beer for her and the kids. Which finally leads me to my question.

How well will a charged keg hold pressure. From what I have read, soda is generally charged and served at a higher pressure than the 12psi i have sitting on my ale. If I disconnect, or just shut off the gas to my ale, will it hold pressure well enough to let me charge my soda. (while maybe hooking the ale up once a day just to make sure it has good pressure)??

Thanks

Steve

Note: I have brand new kegs with brand new o-rings etc.

If your keg seals well, it'll hold pressure forever. Keep in mind that if the soda or beer isn't fully carbonated, CO2 will absorb into the liquid and the pressure will drop a bit.

I concur with Zach - I typically store my kegs with sanitizer from cleaning, under pressure. Then I simply put on a picnic tap and use that sanitizer for cleaning in between.
As long as your beer was fully carbonated and you keep it at the same temperature, you should not have a problem. But if you decrease the temperature more CO2 will go into solution, the pressure will drop, and if you go to extremes you might lose compression on the o-ring seal. (I've only ever seen this when taking a naturally carbonated keg to serving temp without gas connected.)