About a month ago l curled a kind of stable weight in the worng form, my form was wrong, and l curled for a good 30-40 minutes for 3 days, did it harden my arm muscles?l was curling in the wrong form (with my arm extended straight in front of me and flex, at least thats what l thought was good form back then)

Any exercise will strengthen a muscle. Proper form isolates a muscle group to get maximum effect. Improper form may cause undue stress on a joint or ligament. Try using the proper form & u will see improvement more quickly. Also, there r many schools of thought about how many repetitions u should do. Some say u should only do 3 sets of 10 reps at a time, others say do as much weight as u can to failure at 6 reps. Do some reading or talk to a trainer for the best method for you. 30-40 minutes a day on one muscle group seems excessive.