Your tachometer is the best gauge of what is "good" for your engine. Like the post above, don't let your engine lag, or let it run at high RPM's for long periods of time either. Most people with manual transmissions are pretty familiar with the best RPM's for their engine, but Auto people tend to rely on the automatic shifting instead (thus the OD warning for automatic types).
editorial comment: In "general" Automatics while convenient, have removed many peoples understanding of their engines. It's unfortunate with the proliferation of automatics that many can't even drive a manual if situations arise where their lives or others depend upon it.