You can use Sleep Mode as frequently or infrequently as you like...it's entirely up to you.

As to temperatures, there is no standard. The hard drive, motherboard, CPU, PSU, et cetera all have different temperatures and each specific make/model has different acceptable temperature ranges. Thus, your exact hardware configuration is needed to accurately answer that question. I will note, however, that most modern computers will automatically shut down if the temperatures reach critical levels.

John