Curation rewards depend on many factors. In theory it is possible to obtain curation rewards even with only the initial 15SP if you upvote posts with low rewards that go on to make high rewards (and you vote after the initial 30 minute period).
However in practice you need to be generating around $0.004 of upvote to consistently get $0.001 of curation rewards. This currently requires around 50SP.
Curation isn't something worth worrying about until you have a much higher level of SP. Even with 50-100SP the curation rewards would be typically around 0.001-0.002SP, so very, very low.
Instead to build SP I would suggest focusing on:
- Great comments
- Great posts once you build an audience
- Contributing to particular communities which have high rewards (DTube, Utopian etc)
- Contests