Changing config save frequency in 5.1

I've read that by default ESXi 5.1 saves it's config to disk every 10 minutes by default. I'm running off a USB memory stick so would like to lower the frequency of this, especially as the config isn't going to be changing anything like every 10 minutes. How do I do this? I suspect it's buried in the advanced configuration setttings of the vSphere Client somewhere but I haven't been able to find it and Google hasn't thrown anything up yet.