How long is the yeast/trub on the bottom of my fermenter good for?

I just transferred my watermelon wheat to a secondary and would like to start my next batch right on top of the old trub(if possible). How long can I let my primary sit at 68 degrees without risking exposing my beer to infection or the yeast dying. I cant start it for a few days because I will be transferring it to secondary and that will be tied up for 12-14 days.

I would wash it and stick in the fridge. Start with 2 sanitized jars. Fill both half full with water and boil in the microwave. Stick in the fridge until cool (4-8 hrs). Take a sanitized measuring cup and scoop off 2-3 cups from the top of yeast/trub layer into one of the jars. Swirl around good and let sit 15 minutes. At this time most of the trub should have fallen to the bottom. Pour the top half or so into the second jar of cooled, sterile water and seal. Place in fridge.

Now you can be really flexible on when you pitch to your next batch. Its very low alcohol %, and has had a lot of the trub/hops separated from it. It should be viable to pitch without a starter for another week or two. Use a starter after that. As long as it has been more than a couple of days, you want to decant and discard the water on top and pitch the yeast slurry.

If I do it today is it okay to go right on top of it?

I have to agree on the washing idea. It removes a good deal of dead yeast and other things you may not want to transfer to your new batch.

Alright, thanks guys. I still have a half pack of dry yeast to toss in with it. Only a 3 gallon batch so I might just be okay with the half pack of us-05

Half pack of dry would work. You could go direct on top today if you want, but it will be overpitching by some degree. Some will say that has no negative effects at all, but it may remove some "character". I've pitched on top of whole yeast cake twice with good results, but they were 1.080 and 1.102 gravities (Tripel and RIS).

Alright, I'll just clean and sanitize my fermenter and use the half pack.