Back to the OP's question then.
As a general rule of thumb, under blankets shouldn't be left on all night, over blankets can be if they are described as "all-night" Heating up times? how long is a piece of string, but most modern ones have a fast pre-heat these days.
We have a double heated duvet that is controllable for both sides for heat and time. It can be set for 1 hour or 9 hours, but recommends only the lower heat settings (1 to 4 of 6) for all night use. It alleges to be up to full heat in ten minutes. A heated duvet seems a bit OTT in some ways, but I must admit to it being very pleasant when set for an hour on very cold mornings <I dislike heated bedrooms, so much to Mrs P's disgust the central heating radiator is turned off, I do have a thermostatic valve for it, but have never fitted it>
Whatever you buy, buy a well known reliable brand Dreamland are as good as anybody.
Experience; is something you gain, just after you needed it most.
When faced with two choices, simply toss a coin. It works not because it settles the question for you. But because in that brief moment while the coin is in the air. You suddenly know what you are hoping for.
