At home depot they have the "intermatic digital Timer" runs approximately $20
You can program 14 on and off events over 7 days. You can also set it to random. The good thing is that you can set them to turn on and off for as little as one minute at a time.
Kalk reactors do not need much mixing. Figure mix about 2 minutes for every gallon you dose into your system. Thus if your dosing 10 gallons per day, you want to mix 2 minutes at a time 10 times per day.
While you can run them longer. The longer you run them the more you have to worry about wear on impellars or the wear plate on the mag stir variety.