Hi
Have had guppies survive into the 30's for short periods of time. You will have massive die offs in the 40's and then there is the problem of disese problems when they do recover. Certainly wouldn't expect them to survive more than a few hours of these temps.
i keep only tropical fish due to my summer extremes and heat the main pool during winter but I do allow it into the 50's. Most livebearers are the most tolerant but certainly wouldn't recommend it. gary

When I was a kid we had a small guppy aquarium. The heat went off one night and there was ice on the top of the tank in the am. Most of the fish died, but the ones that lived never reproduced again.

This doesn't really answer your question, but it does show me that guppies don't like freezing water!!!
Anne

This seems to be pretty much what I can gather from here and the academic sources, they usually do OK for periods down into the low 50s and then start to get disease and endocrine and immune problems but some as noted above will actually live, at least for brief periods, in the 30s.

Once I bought some guppies during winter and until I got home the water was so cold that I found them all floating with their bellies up and "dead". But they recovered in warmer water slowly and lived just fine...

Here is a link that might be useful: guppy

Well I've have guppies all year round outside lowest I've ever had them servive is 50 degrees farenhight did not show any distress but then again I do add crushed rock salt 1 tespoon disolved to every 90 liters to keep ick and spot also fugs and in summer temperature can exceed 90 farenhight