What is the minimum temperature yeast will be active?
It will even rise (very, very slowly) at refrigeration temperatures (4C, 39F). For it to completely stop rising you need to freeze it (or very close). A lot of recipes call for the dough to rise overnight in the refrigerator. Depending on the dough, it may it may in fact completely rise in the refrigerator overnight. Long rises create more flavor than short ones, often speed is not of the essence. Frozen dough will start (slowly) to rise as soon as it starts to thaw.

Never judge readiness by time. Temperature is only one of many factors that will vary from loaf to loaf.

Volume is the main indicator of readiness (usually double the "pre-rise" size). Also, there's the poke test. Stick a couple of fingertips a centimeter or so into the dough. If the indentations remain after your fingers are removed, that's an indication that the dough has risen enough.