George wrote:
how long does it keep after opening the bag?

TIA


That entirely depends on local hunmidity.

I had a bag I got this summer for some odd jobs. Left outside in the old
landrover.

The outside and top had caked, but inside it was still powdery. It set
Ok when I had to put in two half bricks to tidy up some decorative
brickwork.

If you seal it against moisture,. it lasts forever. If it gets rained
on, its a solid lump the next day ;-)

If its still powdery, its OK.