Depends on both the inside and outside temps, the amount of energy heating the inside, and the area exposed to the lower temp.

Assuming a square box of plexiglass (0.17 W/mK thermal conductivity) with sides 25cm, to maintain a temperature of 25K (read:celsius) over ambient (so, if it’s 0°C outside, it’s 25°C inside. If it’s 25°C outside, it’s 50°C inside), the inside of the chamber must be heated with a power of:

0.17 * (0.25^2 *6 (surface area of our cube, in meters squared)) * 25 (the temp delta we want, in kelvin) / 0.0016(thickness of the plexiglass, in meters. Here I took the common 1.6 mm ~= 1/16")

Dimensional analysis: W/mK * m^2 * K /m = W

The result is 996W which mean that for the inside of the chamber to be 25K (again, read that as celsius) over the ambient temperature, so 23°C when it’s -2°C etc, the total power your printer must use has to be about a kilowatt. Much of that power is already being dumped by your hotend and your heated bed, as well as your motors if they’re inside the enclosure. I suggest hooking your printer to a kill-a-watt or similar to be able to see how much it’s already using, and deduce how much you would need.

Once you know, you can tell the temperature delta (again, in Celsius) using the following formula adapted from the one above:

∆t= (power you measured) * (thickness of the plexiglass in meters) / ( (side length in meters of your enclosure)^2 *6 * 0.17)

(Be careful about the parenthesises)

The result is the number of degrees Celsius above the ambient temperature your enclosure will be at all times (actually not, since some components inside will dump less power if the inside gets hot, for example the heated bed, but it’s a useful approximation). If you get a result of 30, and it’s 15°C outside the enclosure, it will be 45°C inside.