Do you mean stacking from tick to tick or stacking from being surrounded?
Rise to the Challenge has two parts to it. IIRC, One is a larger +Regen buff that is capped at one target, and the other is a smaller bonus, capped at 10 targets. Each bonus lasts one server tick over 10 seconds, and refreshes itself every 10 seconds. During the one server tick overlap, you have all the bonuses from the previous cycle, plus the bonuses from the new cycle.
I'm sorry, but I can't say how big those bonuses are - I don't have access to numbers right now. But you get ONE big bonus, and several small bonuses.
@Roderick
Incorrect. It does not last 10 seconds. With RttC it refreshes every second. That is mainly why the taunt aura sucks. You do have a blip of overlap for server ticks where it gets double stacked. That is where you are getting you 1/8 number from. .125 seconds of overlap every second.
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Rise to the Challenge has two parts to it. IIRC, One is a larger +Regen buff that is capped at one target, and the other is a smaller bonus, capped at 10 targets. Each bonus lasts one server tick over 10 seconds, and refreshes itself every 10 seconds. During the one server tick overlap, you have all the bonuses from the previous cycle, plus the bonuses from the new cycle.
I'm sorry, but I can't say how big those bonuses are - I don't have access to numbers right now. But you get ONE big bonus, and several small bonuses.
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I have no clue where you're getting your 10 second buff number. That's way longer than any toggle power's individual effect lasts. The power itself activates every (1) second. The "primary" (re: non-stacking) buff lasts 1.125 seconds and the "secondaries" (re: stacking) buffs last 1 second, but, thanks to the clock interval not evenly dividing a second and small amounts of lag, you could actually see the buffs themselves stack (appearing to double up). You can actually see this phenomenon on many different toggles. You won't actually see a really effect from it however, mainly because they actual period of stacking is so insignificant as to be completely and totally ineffective. It will flash 16 stacks every once in a while and you'll see huge hp/sec numbers in your combat attributes, but, I assure you, you will actually experience the lower, more common number.
Let's clarify the OP : by RttC stacking I mean the small period of time during which RttC gives twice as much buffs as it should.
Redtomax lists it as 0.13 (I can't believe I didn't check this site in the first place), close enough to one server tick, and my testing showed me it's giving a ~13% buff to RttC regen. I am wondering if someone knows exactly how it works, but that's just curiosity. Most likely, it's working exactly like it looks like it should, giving a straight 13% (or 13.2%) increase.
Careful, Nihilii, you're looking at the wrong regen bonus:
Rise to the Challenge
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Activate period 1 seconds
* Regeneration +0.25 for 1s If NOT on a PvP map
* Regeneration +1 for 1.13s If NOT on a PvP map
Effect does not stack from same caster
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The one you picked was the first target bonus which is set not to self stack. The bonus you're looking for is the other one, which should last just as long as the tick rate (ie: no stack).
I once asked Arcanaville about the overlapping buffs on the Tanker forum, you can read my post here and her response directly under it.
It's not "lag" (ie: network) that's causing the overlap, but the internal clocks of the game.
Ah, thanks !
Well, reading that left me even more confused. I've never seen my defense moving on my SD, Phalanx Fighting looks like it's exactly perma.
Are we 100% sure the numbers on Redtomax are correct ?
Edit : scratch that, I'm being stupid again and thought you were talking about the passive part. The other part does indeed vary with teammates.
If I understand this, that would make the duration of the stacking effect RoundUp(1/0.125)*0.125, right ? Yet that can't be right, because it's = to 1, and RttC definitely stacks.
Using 0.132 instead, that would give a 1.056 duration. It doesn't seem like 0.056 is enough considering I did consistently get numbers over +0.1, but it could be that I didn't run enough tests... or that I got the equations wrong. Again. 
Just out of curiosity,
Has anyone managed to find out how does RttC stacking affect the regen ?
My gut feeling was that it was (just like Invincibility) stacking for 0.125s (or possibly one server tick ; that would make more sense, wouldn't it ?), and running a few tests against rikti monkeys, the numbers I got were significantly higher than my supposed regen, more or less matching a 1/8th boost.
That being said, it's a rather crude way to test (tiny samples, using a stopwatch...) so I'm wondering if anybody knew exactly how it works.