There are two programs for collecting bee venom;
First is every 3-4 days by colony, for 50-60 minutes two times in a same day. At least 25 min pause between collecting bee venom on the same hive.
Second is every 10'th day by colony, for 3 hours of collecting bee venom on hive.
About 20 000 stings you need to get 1 gram, which is by colony 10 grams or more, in season. That mean roughly that you need 3-4 months to collecting on 75-150 colonies, to get 1000 grams in season. Depend on how strong and big your colonies are.
With collecting you don't hurt even one bee. Thru last 20 year researches show and they confirmed that often emptying and again filling a venom bladder is good for bee, her organizam, and it will prolong life of bees.