Pretty good heater pack. We had 8 of them in our trip, all worked. Although the reaction start speed might wary drastically. Some packs went off in less than 30 seconds, but some laid doing nothing for solid 5 minutes. But once it starts going 12 min would be enough to warm up your food. Note, cardboard to cover it is not included.
Heating time of 12 mins given on packaging not very realistic, ( possibly for rations which come in cardboard packaging and is used in the cooking process ). Not having this I experimented with cooking my british army pouch meal by inserting heater bag into a suitably sized jiffy bag ( plastic bubble wrap on interior which can be reused ) . Still needed to leave for nearly 20 mins for best result and also spun around pouch after 10 mins to ensure both sides receive direct contact with the heat pack ( take care very hot ). Satisfactory result.
My goto emergency heater
Took time to activate the heater and once it got going it was fine. A portable, disposal cooking bag. Genius invention!
The heaters worked but took much longer than the 12 mins stated before they even started getting hot. We had limited time so took the rations out after 20 or so mins, they were warm to eat but not hot. The heaters continued to give out a very high level of heat, they were too hot to hold so I think if we'd left them for longer our food would have got hotter.