Tiling kitchen floor.

I use the powder stuff, like you say it works out a lot cheaper. I seem to remember I had something like 30 minutes of working time before it started to go off. That will vary depending on the make etc. I also recall there were admixtures you can add to slow it down, although usually 30 mins is adequate.

I mix up in a bucket using an electric drill with a mixing paddle like this:


They take the hard work out of mixing but they do need to be kept clean, especially between mixes, if you transfer any old 'going off' adhesive into the new mix it can cause the new batch to go off much quicker.