An average battle, one with NPC reinforcements helping out, I get slightly lower returns because the battle obviously ends quicker (around 15-30 mins). Some of the longer ones, like 1-2 (even 3) hours, I'll get an assessment in between waves if it looks like it'll be a long one. Each assessment I get around 1.3-1.5k. So, on the longer ones I average somewhere around 2.5-3k. You also attain a baseline amount of xp/min depending on how long the battle lasts and your current medal.
Really it doesn't matter if they're getting as much xp as I am helping out. The point is that just sitting there spamming songs is the reason we have next to no areas under our control. The more areas we have = easier it is to win battles = easier to get xp. Amirite?