Lisa, I think with your strong academic background and work experience, getting interest from relevant people should be straightforward given the advice earlier. A traditional marketing role should also be attainable via regular channels.
The bit that is trickier is 'the right time' part of your question. I am guessing its that you want to get on board with a new opportunity during its formative phases so you can apply your startup marketing skills, etc. For this you need to be in the startup scene, know other people doing startups and provide support to those without the facility to on board marketing specialists. Being active in this forum is a good start. Also there are meet-ups, hackathons and other startup focussed events. Where appropriate, try and get involved with those. Its the corridor discussions and coffee breaks at these events where you'd do the real work of aligning to the right people, but at the 'right' time.