Above all, I think it's vital that ST survive and evolve to stay relevant to an evermore demanding audiance in an evermore complex world. If it takes 20 years to do that, so be it. But I personally think that would be a mistake.
Waiting 20 years could potentially disenfranchise those 2nd-gen fans who don't necessarily have ST "in their blood", as apposed to someone like me who thinks that a bad episode of Trek is always better than the best episode of (ENTER THE NAME OF ANY REALITY TV SHOW HERE). Either way, I have no need to get cable again until another Trek show comes out - Netflix and Moviestop satisfy my needs in the meantime.
I think the only important time factor is the one dictated by the Abrams movies. I think it would be wise for them to start thinking about a TV show now that will follow. Whether the next show will compliment the Abramsverse or add on to TNG's history, I think it would be smart to wait until Abrams has left the building. Besides, if he ends at 3 (maybe around 2016), then that would already put us at 12 years since the end of ENT. That's more than enough of a cooling off period for the over-saturated 24th century.
Abram's rule will leave us with one problem: Assuming no more alternate realities are introduced, future producers will be forced to choose a show with no Romulus, or one with no Vulcan. Writers are creative, so I'm sure they could make it work, but I think they've allowed Abrams to sacrifice a lot for his ST-Reboot's success. Is it worth it? Hopefully.
Lastly, I think ST story-tellling needs to evolve for modern TV. Instead of the old minor sub-plots loosly weaving each episode into some barely-relevant chronological order, they need to have a big idea. Specifically:
1. There should be a clearly outlined arch from beginning to end, something that tells us why this ST story is important enough to be told, yet contains enough mystery to keep us intrigued.
2. They should consider trading in the traditional 24-episode season for a 10-part serial mini-season, with each season serving the equivalent of a single novel in a collection series. A show with a defined outline of 10 episodes times 5 seasons might be attractive to places like HBO (especially coming off of a huge box office hit like Abram's Treks).
3. They shouldn't limit themselves to a single starship or starbase. If budget permits, they could consider a larger, loosely connected ensemble cast set in various locations. I'm sure the writers would love this as it could facilitate more complexity and diversity in the story.
4. No matter how the Abram's run turns out, I vote that he got the tech, look, and CG of Trek to finally rival (or surpass) the bar-setting Star Wars quality. Everything from the massive look of the Budweiser plant/ engineering decks to the Warp jump effect are instant favorites for this Trekker - any new series should try to preserve that.