The more nights involved usually the lesser chance of a suite upgrade.

But as nachosdelux has mentioned, it depends on what else is going on. For example, is there a convention during your stay? The Ren often books 100% full during conventions as it's literally next to the south hall of the convention center & often suites are booked by corporations. There's also time of check in, front desk clerk, if it's cloudy or sunny, etc.

Cheers.