Well I guess it depends on what you are going for; if you want them to be a straight up space faring race (As in far more space faring than we are, where space voyage is nearly as casual as a boat cruise ) we'd need at least 500+ years post season 4 I'd think given that pony society with it's weirdly diverse culture and tech levels gives me a very Victorian kind of feel. Using that as a measuring stick and our own current meager jaunts into space, if we want the pony's to be at least halfway into exploring their own solor system, we'd need at least that many years.

 If we just want the magical and tech advancement Then a handful of years like two or three hundred would do fine. Enough for the introduction to alien species would make a massive leap in space faring tech likely. Like how the space race forced us into space so many years ago when the tech was super primitive. With advanced alien allies and this new need to be apart of the great space culture I could see pony race be )

And why not a prince I guess. We have seen nothing in cannon to support or deny that possibility. I'm not surprised by the lack though, given the show is designed with the female demographic in mind.