How long can a hermann tortoise go without a heat lamp?

I am pet sitting for the week and the tortoise heat lamp burnt out on wednesday afternoon. I can't find a replacement anywhere in the house and none of the local pet shops stock them. They are back on Saturday so how long can the tortoises go without heat? They have a UV tube but im not sure if this is enough. When i fed them today they seemed really slow & were obviously cold as they were huddling under a log ornament thing, I just don't want anything to happen to them! It's been over 24hrs now!!