Two ways to take your question of "how many times you done it"...
I've re-vegged the same plant up to 8 times, or so. Some claim fears of strain degredation or a weakening of the genetics, growth or effect...but I have yet to encounter this problem.
-or-
I've re-vegged perhaps a hundred plants over the years. (no, not all at the same time) Tiz about the easiest advanced technique I've ever come across. :thumbsup:

Bagseed is a great learning tool for beginners. But using an unknown, possibly unstable bagseed might not give you optimal results for growing as a medicine, or for use in advanced techniques like re-vegging and femming.

Collect the (femmed) pollen, remove any plant material and dry it out for a day or two. (away from your plants, away from fans, and don't sneeze it all over the place)
Once it dries-out a bit store in a tightly closed, water-tight container in the fridge. (round containers are watertight, square ones are airtight...I learned that selling TupperWare 30 years ago) Moisture will kill the pollen.
The longest I've stored pollen is about a month. Likely it can last longer, but I've never felt the need to push it. But if the process was done properly to make and collect the pollen, you can pollinate any female with it, and all the seeds will be female.
"Paint" the pollen onto the pistils of the plant you want seeded with a tiny artist horsehair brush or a Q-Tip with most of the fuzzy stuff removed. Use care and avoid any 'unauthorized release' of pollen. I 'paint' lower branches to preserve the integrity of the larger growing tips. (the buds)