i know that it varies depending on what strain of the plant but i was given a a mixed bag of seed so cannot give a set time.

what signs am i to look out for on the plants to tell me they are ready to chop?

taa

Hi flip have a look at this some good info

They usually put the bulk of their weight on in the last couple of weeks, and you can also judge their readiness by checking the trichomes with a magnifying glass or loupe.

Look here: When should I harvest my plants? and here Harvesting & Curing