18 hours fermentation of Biga
1: Done is when you are happy with the result, less than 18 hours means less fermentation, more means more fermentation, it all depends on how much flavor you want to get out of it. 18 hours is approximate and as you say there are many factors. For a biga I'd use my nose, you will get a feel for when it is ready. Much more than 18 hours might get a bit strong.

2: Refrigerators are great tools for bread baking because they slow down the processes without stopping them or killing anything. You can ferment your biga/starter, or prove your bread in the fridge, it just takes longer. You could leave it out for 10 hours and then 16 in the fridge for example, or start it 2 days early and keep it in the fridge the whole time. A refrigerator is also very consistent in temperature, whereas room temperature varies from day to day

Keep in mind that the starter fermentation is about flavor development, and is an optional step. I always use a starter (a starter is a biga you don't ferment, you use it to get a good, healthy base for the rest of the dough) when making pizza dough but I rarely ferment it and I still get excellent results. Often I will make the dough and then refrigerate it for a day, I get the flavor development later in the process. So if you have 6 hours you can ferment the biga do it for 6 hours, it's not critical.