Hi RFMad,
From the Game Guide Chapter 9 about hiring:
Reworking Neighbors Facilities:
Explaining the time between working a friends facilities before you can rework that same friend's post again.
To simplify the answer it was originally stated that you had to wait a day. Meaning that if you spend at least 8 straight hours in a day without playing FarmTown (when you sleep, eat breakfast, etc, most people will fall in this category) you will be able to work your friends facilities at least once per day.
If you wanted to work them say twice per day.... when you have stopped working friends facilities, you must also have an 8 hour break from working ALL friends facilities, no matter whose they are!
Example:
IF at Noon/Midday you stop working ALL friends facilities and take a minimum 8 hour break where you do NOT work any facilities at all for any of your friends.
After 8pm you will be able to work ALL your friends facilities, including the ones you worked before Noon.
NOTE: If you are only one second too early this will not work!!!
There is a countdown timer that resets to 8 hours each time you work facilities. There is one for Regular Facilities and another one for Service Facilities.
To help you with this, there is a tool in your toolbox called Show Times (looks like a clock). Click on it and then Look in the Gamewide section. Toward the bottom are the timers for Facilities.
Always reload your game before you want to rework facilities and check the Show Times!
If you want to be 100% sure that you have had a long enough gap between working each type facilities make sure that before you work any at all for any friend, that you check that you have a green check mark for Rework Facilities and Rework Service Facilties. Then, and only then, can you rework the facilities.
If there are no green check marks showing you are too soon to start working them and need to wait longer. The time showing will help you know when that will be,, you can mouseover the times to see more informtaion, but always check for those green check marks.