Don't bother waiting, warm it up now. Especially with a heff, where you want some of those esters, you're not in danger of producing off-style flavors, and, with it already down to 1.020, you're unlikely to notice any flavor change at all. Also, once you get it up to the low 70's, give the closed fermenter a couple of swirls to rouse the yeast back into suspension.
There are several cheap, easy things you can (and should) do to help with those temp swings, too -- wrap it in a blanket, stick it in a bath in a 15-gallon plastic tub (10-12 gallons will resist swings a lot better than 5-6), or, both!