Turn it on during installation of software and switch it off at other times... Should be okay... Personally, since I rarely install stuff, (and when I do, I get software that I trust from original website), I dont use the realtime protection... Instead, I simply run the scan regularly to confirm absense of spyware...
Do ensure you have antivirus and firewall running, then you dont need to have Spybot's realtime protection running all the time...
Arun