A couple of days ago, sarah m asked this question, which is an important one to know the answer to if you want to be a good neighbor:
Which buildings take the longest to regenerate income (i.e. which buildings should we tap to provide the most benefit to our friends?…I try not to tap Simpsons House, the Brown House etc. as I know they regenerate super quickly)

First, there are some buildings you shouldn’t tap on. Sure, they give you a small amount of cash and XP, but tapping on them doesn’t help your friend at all – they’re Channel 6 and the Open Air Stage. If you see a dead crop at Cletus’ farm, though, tapping it may revive it. And tapping on the Simpsons’ House, the Brown House, the Kwik-E-Mart and other buildings that generate rent quickly isn’t helping your friend as much as you can. (You can graffiti the School, Moe’s, the Police Station, and the Town Hall as well, but that’s for another post.)

The only reason you’re able to tap on a building is if it’s ready for your friend to collect rent – but they haven’t been around to do it. Your tap effectively collects the rent and puts it in the bank for them so not only is it there to collect when they go into their town again – you’ve also restarted their rent collection cycle while they were away. Which is why, if you really want to help your friends, it’s good to tap on the buildings that take the longest to regenerate.

Here’s a handy list of the buildings (and a decoration) that take 8 hours or more to regenerate:
2 days: Burns Manor; Krabappel’s Apartment
24 hours: Duff Stadium; Pimento Grove; Springfield Penitentiary; Springfield General Hospital; Volcano Lair; Holiday Tree
16 hours: Fat Tony’s Compound – Phineas Q. Butterfat’s; Springfield Library; Springfield Observatory; Orange House
12 hours: White House; Hibbert Family Practice; Sprawl-Mart; Santa’s Village; Sleep-Eazy Motel; Mapple Store; Wiggum House; First Church of Springfield; Skip’s Diner; School Bus
10 hours: Asia de Cuba; Legitimate Businessman’s Club; Howard’s Flowers; Springfield Cemetery; Springfield Pet Cemetery
8 hours: Swanky Fish; Springfield Coliseum; El Chemistri; King Toot’s; Jake’s Unisex Hair Salon; Sir Putt-a-lot’s; Springfield Buddhist Temple; Blue House; Aztec Theater; House of Evil; Mausoleum; Super Collider

Thanks to everyone who helped to fill in the gaps that inexplicably appeared between writing this post and posting it here. That should mean that any building missing from the list takes less than 8 hours to regenerate.