I don't bother with a screen saver ever, I'd rather my system went into standby. But I have mine set to generate the VRDB when screen saver is running.
So effectively it will never run automatically, I do regular maintenance tasks and every two weeks as part of those tasks I manually run the VRDB. I do it at a time when I am not going to be using the computer for a while. It only takes a few minutes 5 minutes tops. Your first one will usually take some time but after that 3-5 minutes.
This is obviously dependant on your system, cpu, ram, size of data to be checked, etc.