It is 12 letters. Second thing is that I used random string generator, with both Lower, upper, numeric,... So bruteforce would be only way, but on so large password I doubt it would be worth trying.
(26 lowercase + 26 uppercase + 10 digits) ^ 12 = over 3 thousand billion billion permutations (for comparison, that's somewhere around the number of grains of sand on planet Earth). If it was in fact generated randomly, it's completely unbruteforcible.
I think anything more than 9 characters is considered pretty strong.
It's somewhere around there, but it depends on the details. For example, if it was only lowercase letters and 10 letters long, and if it was for a wallet that doesn't use
key stretching (e.g. MultiBit Classic, Electrum), then it would cost me somewhere around $2,500 - $3,500 USD in compute resources & power to brute force, and it could be done in a few weeks. It might be even cheaper to create specialized software that'd run faster and cost less to run, but that's harder to estimate.