This is discussed thoroughly on DAT's forum. It seems to be split on just how long, but the prevailing wisdom seemed to be around 12 hours tops, if I remember correctly. So basically don't leave them mixed anymore than that.
This leaves a couple options...
Some people will use 3 syringes during the day, loading them all in the morning with an air bubble between the two peptides...when you are ready to shoot each one, just give it a quick hard shake to combine them, push the air out, and have at it. This gives you 3 pins a day but requires more trips back into the vial stopper, which dulls the needle. Not an issue if you don't reuse needles, but many people do.
Others will mix up all their doses to be taken within a 12 hour period. This will work if, say, you work out in the evening...one dose prior to working out at 8 pm, another before bed, and the final when waking up at around 8 am.
Personally, I think the 12 hours is probably overrated so I don't really plan my life around it. Even if it degrades a bit, it is still good. Might just use more or something for that last dose.