Ok physics time:
Really well made glass melts at 1923K
Since lasers are pretty much just 1 wavelength, we can fine-tune the mirror for that frequency provided we know what laser the bad guys are gonna shoot (like maybe some alien faction all use the same lasers so you know from before).
This gives a 99.999% perfect mirror.
Glass heats up at 840J/KgK. imagine we make the glass pretty thick, because we also want to shield ourselves from stuff like ordinary bullets, so say it's 1cm thick.
Now the laser is gonna be pretty small in area. It's gonna be anywhere from a couple millimeters to a couple micrometers. Let's assume something large, as a hole the size of a couple micrometers in a hull isn't even going to get noticed, let alone destroy the ship. So we take 3mm diameter. This gives us, with glass being 2500kg/m³, and the surface area being (pi*(0.5*0.003)²)*0.01*2500 = 1.775e-4kg of glass. Now if we want to heat this mass of glass to it's melting point, we need 840*(1.775e-4) = 0.1491 joules per kelvin we want to heat it. Assuming the glass is 0K due to the coldness of space, we can tank this amount 1923 times, giving us 287 joules.
So if the glass recieves 287 J, it's gonna bust due to heat. HOWEVER, we can take 100000 times that due to the efficiency of our mirror being 99.999%. So that gives us 28.7MJ. That's actually pretty decent.
THe US army's current lasers, at 30kW, would take 957 seconds (or 16 MINUTES) to melt your shield.
EDIT: Forgot to multiply by the density of glass, whoops.
