Found a way to make cap times variable. You can use whatever you like as the input for changing cap times. I just used another point as an example.

Cap times only change if there's no progress on the point that's being capped.

That means that if you've already got people capping the point that is having its time changed, the new time won't update until they all get off and the cap percentage goes to 0.

Better for logic simplicity (and gameplay imo).

Due to discussion about whether it's faster to stay on or get off and then on the point. I have made this graph:

https://www.desmos.com/calculator/3moeds60ct
a = Old amount of time needed to fully cap the point
b = The percentage to which the point was capped before the new cap time was applied
c = Amount of time team has capped for before the new cap time was applied (a*b)
d = decay time (90*b)
g = New amount of time needed to fully cap the point

so.

Time if you keep on the point = y = ax
Time if you get off the point and get back on = y = gx + c + d

The black line is when it is at 100% cap

There are very few times when it's quicker to get off and then get on again. Even with an old cap time of 10 seconds and a new one of 1 second. If blue team has capped even 20% of the point it's quicker to stay on it.