Yes, the basic "you know when" unless you're me. After a while of thinking I really should be driving a stick, I've now dropped my G6 into manual mode only to forget and wonder "gee...why isn't it shifting?" when I take off again. AND I just did the same today playing with the TapShift for the first time in an '09 LTZ 2.4L. I wondered why that Ecotec was at 4k rpm at 15-20mphJust listen to the engine. When you hear it tightening up, it's usually a good time to shift.
generally speaking if it was a 2.4 with a stick I'd say you can shift before that in normal driving. I'd say 2500 is fine or even lower unless you need power. When I had a Vibe with 1.8 130 hp engine, i think that's where I'd normally shift and that was a pretty anemic engine. On a Ranger with 2.3 140 hp engine I think I was shifting close to 2000, but it's been a while I may not remember correctly.I just go when it normally changes in automatic.. around 2800-3000 depending on the power I need at the moment.