<p>I am trying to print something that might take 15 hours. I don't want to risk my printer so if I print for 15 hours, what is the worst that can happen? So far, I haven't printed anything for more than 5 hours.</p>

Correctly level your bed. Seriously, that's the answer. PETG does stick well, but it only gets difficult to remove if you're smashing the first layer against the bed with a nozzle that's way too close. With the bed leveled properly - using feeler gauges or test prints and a sub-0.1-mm-precision caliper - I have no trouble taking PETG prints off a buildtak-clone bed. Glass should be easier.
If you already have PETG stuck to a build surface you care about and don't want to risk destroying it, try heat, or alternating heat and cold.