I am going to be neutral on this. Whatever make the most people happy is fine with me.
I have always been a influence victory player. At least 3X more influence victories over conquest. I like the combat but I hate the mop-up when playing on big maps. In GCII I was Jack the Flipper. The flip was tied to influence of one planet over the other. Once your influence had exceeded the other planet by an amount that I have forgotten the planet would flip regardless of turns. I recall as few as 5 or so turns and as many as 20 if the enemy planet was high Q with large population. Building culture bases was also a major factor. By my recollection it was a lot easier to flip planets in GCII.
In GCIII I find it more difficult to go after planets to flip them. I have won influence victories but the flips I got were from my overall influence expansion and not my my deliberate effort to flip a specific planet. If I build a culture base near another race's border it seems to not have the dramatic effect that it did in GCII.
In the current Gigantic game I am playing, I hope to win an influence V. I am playing at the normal level and at about turn 250 I only control about 40% of the galaxy against all five opponents. Since they fixed the culture bug a few weeks ago I have not lost any planets to flips and I haven't flipped many since I have been playing around with the Battle viewer. In this game I got extreme worlds tech and just for S&G I colonized a PQ 10 frozen planet just 2-3 hexes from two well developed Iridium planets, I fully expected to lose the planet but I was going to see if I could save it. Sure enough I immediately get the 10 turns message and I start sending constructor to see if I can get them there in time to see if I can save my planet. I soon figured it is going to take too long to get a SB in place and the planet will be a goner. I rushed a consulate and was just waiting for the inevitable. What do you know, about 5 turns later my planet had carved out a tiny perimeter and was safe from the flip, for now anyway. I have since colonized a couple of more planets in Drengin and Yor zones of control and they both set their own small zoc immediately, (they were not as close to rival planets as the first one.
My point is, since they fixed the culture bug, if you have made any kind of effort to build a base of influence, by building influence improvements and researching a few culture techs, my experience shows that your planets will not be easy to flip, nor will it be easy for you to flip another race's planets until your tide of overall influence starts to inundate them.
As for increasing the time from 10 to 20 turns, it probably doesn't matter. I am not sure how smart the AI will wind up being in GCIII but in GCII and so far with the place holder, the AI has one solution for culture invasion, War. If you are stronger diplomatically and militarily, they will not declare war to save their planets. I have never seen more than a halfhearted effort by the AI to fend off a culture attack. That frog looking race in GCII with super growth was great at trying to flip your planets but they were all offense and no defense. Once you got ahead of them in culture and military they would fold. Unless the GCIII AI is, not just smarter, but dramatically different in their philosophy, the only difference in 10 or 20 turns will be the longer wait.