You can re-use it indefinitely, and over time the more you re-use it the better it will get.
I usually re-amend my soil with:
- 5-10% worm castings per total soil volume
- 5-10% perlite per total soil volume
- 1/4 cup dry amendments per gallon of soil (Espoma, Gaia Green, etc, whatever is available near you)
I don't test my soil between rounds, if the previous round seemed a little overfed then I will go lighter on the dry amendments, and vice versa.
If it starts to get heavy or muddy after a few rounds, I will cut it with a peat-based potting mix to lighten it up.