My pmd-661 using AKG 460s with p48 was taking two sets of 4 AA's to do any 2 set show. If the first set was any longer than 60 minutes I would automatically swap them out.
That said, I'm not a tester type person, so I never actually tested how long they go with that config. (probably check the 661 thread digi-gal is referencing.
As for digi in, I have gone close to 10 hours on a pair of AA's with digi in from a HDP2. I typically prep for 6-8 hours if I am doing digi in and swap out when the 661 shows 1 bar.
As for Line in, I haven't done that enough to have an answer for you.
Now, I use a Tekkeon 3450 (NOT recommended by MFR) as digi gal mentioned. So far, I've had no problems with it unless I forget to turn it on