Aside from the very popular PS's, I rarely have problems up until about 45 days out.
Most people (aside from the loonies that hang out here
) generally don't plan that much until they're about 30 days away. Although, Disney now warns people with messages on the My Vacation on the Disney website as well as a mailing to suggest calling in advance. So, as was suggested, as soon as you firm up your schedule, then make the PS's as soon as you can.