Since I can't see how deep the pond is, and it's in the middle of summer, I'd start with 1/2 hour for the first 3 days, increase to 1 hour the next 3 days. All you have to do is run one day at too much and you can kill fish..... Be conservative. If water temp was in the low 50's I'd say start it and leave it run, but not now.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).