Does setting a higher refresh rate on my monitor shorten the lifespan of my monitor? I have a viewsonic E70fsb monitor. Thank You.

Personally, I do not think so. Refresh rate works for CRT monitor and what it does is help the monitor to display better and faster so that you won't feel your display is flickering.

Don't use an unsupported refresh rate and it should be just fine. Check the manufacturer's specs to see what is supported (don't trust what the OS tells you) if you are really concerned.