It be sorta tough some times, but ya kinda get a "feel" for it. I pull a bag outa the water an let rest fer a couple a minutes, then feel round the middle flatten out the meat some, ifin there be a cool spot I have a contact thermometer that I use, ifin it's real close then I'm good ta go, cause once ya start mixin up the meat the hotter stuff will warm up the cooler stuff.
The last gig I did I had ta use the microwave ta get started servin, we had planned an hour ta get the frozen meat warmed up in boilin water, the stove was junk, so low powered we weren't gonna make it so tossed a couple a bags in the nuker, then kept turnin em an flippin em bout ever 3 minutes so they wouldn't overcook, luckily the meat had been very juicy before an didn't dry out.
Refrigerated bags (depends on the amount a meat yer warmin to) take bout 15 ta 20 minutes. Frozen I figure 45 minutes ta an hour. All this depends acourse on how much ya got in that bag, the horsepower a yer stove an the amount a water ya got in yer pan. It takes a bit a learnin, but sure is a great way ta warm up the meat with a bit a practice.