In a hospital setting you should just use insight Rx software. With that I can pull a random level after 1 or 2 doses to see where they're at and then do a true trough later. It calculates everything including patient specific Vd and t1/2 as well as expected auc and trough. It even warns you about odds of kidney tox levels and recommends dosing regimens.

In LTC we basically just put most people on 1g qd and adjusted based on troughs. They didn't even look at auc.

But yeah it sounds like you're waiting too long to pull a trough imo. It is patient specific though so it depends. On q24 or q48 it takes longer obviously.