How fresh does the cheese have to be?
Depends on the amount of initial bacteria, affinage temp, strength of proteolytic enzymes, etc. In general, you can culture species from a rind effectively for years, and easily for up to 6 months. But it varies. If you have a culture sample and do an agar smear, even 2-3 bacteria can quickly grow to a very sizeable colony.
What's the best method for extraction?
Commercially, you try to isolate the purest culture you can on an agar smear, repeating until the colony is sizeable, then you multiply the colony in liquid nutritional agar, centrifuge, and store the sample. Storage depends on species. Often, freezing with glycerin is used so the frozen water crystals do less damage to cell walls.
At home, if you want rind flora on a cheese, you can try the method Francois posted, which is to take a piece of rind, start with a tomme recipe, and then introduce the rind flora to the cheese.
How often can you be successful at this?
Hard to say. Microbiology isn't easy. Isolating pure culture isn't easy, either. You can have contamination, competing species, bacteriophages, penicillin molds that kill bacteria you want, etc. In general, if you have a good lab, and a lot of technical experience, you can isolate the cultures you want, both from cheese and rind.
Is it too much effort should I just go for the packaged culture?
That's something you need to figure out. Some cheese factories keep a lab on premise and culture their own starters, making sure to follow proper species rotation, bacteriophage control, etc. It's expensive... a bit cheaper if you want just the rind flora because it finds its own equilibrium. If you wanted to produce lactic acid strains, that's more difficult because you can't just culture multiple strains all at once. Each strain has specific growth characteristics. For example, for meso bacteria, some multiply more rapidly at 80 degrees, and others at 90. If you want a specific blend, you have to culture separately and blend to get the profile you want.
If you want to replicate rind and don't have a lab I would suggest:
1) take a piece of existing rind that has not dried out and exhibits active growth (15-45 days old)
2) Puree rind with some milk and add a little nutritional yeast.
3) Let that sit in a warm (90 degree) place for 2-6 hours. Strain out solids.
4) Take the culture and freeze in small ice cubes. If you make cheese often, can also store in the fridge and reculture the mother solution.
Then when you make your cheeses, take out a cube, dissolve in some water, and spray your cheeses with the solution.
If you have a lab, there are better ways of going about it using agar and centrifuging. Oh and the simplest way is to take a piece of rind that has blooming molds/bacteria and put it in contract with the new cheese. This works well for candidum strains.