How much heat conserved depends on many other factors. Size of GH, exposure, glazing (number of layers), soil temperature. If everything else is optimized, tiny air leaks will be okay.

You can get an idea by putting a max/min thermometer in there, shaded from sun. Place another outside at the same height. Compare the two minima. Also, get a soil thermometer and monitor how it relates to minimum temperatures inside the greenhouse. As a VERY general rule, I find that if
X is the soil temp.
Y is the outside low
Z is the GH low

Z = k(X-Y) + Y
where k is a factor dependent on GH type, location, exposure, weather conditions (clear vs cloudy, calm vs windy), soil moisture, mulch, and depth of soil temp. probe, among others. You should try and get a general idea of what 'k' is on clear nights, what it is on cloudy nights, etc. It will change a bit with season but not much, so start measuring now :)

for my poorly insulated, single layer PC greenhouse, k is about 0.25 to 0.3 on clear nights and 0.10 to 0.15 on cloudy nights.

Yesterday morning, the low was 38 degrees (!). Soil temp. at 6" depth was 66 degrees. G.H low was 46. k = (46-38)/(66-38) = 0.29. This approximate trend has held true since installing the GH.