From a business perspective I think it's important to move from thinking about "elapsed time" to "value created", especially when considering co-founders with different skill sets. In my experience the two are only very loosely correlated.

How would you barter between product development and sales?

How about if the sales person worked a day a week but closed ten times as many sales as the full time dev?

What if you got a co-founder who investors trusted from prior experience that drove them to actually invest?

How about someone leaving college versus someone who could earn several $100k on the open market?

Apparently[1] teams tend to do better than loan founders, and ultimately whatever you put together has to be tuned to what would motivate the other members of the team to stick around as long as you need them to in order that value is created. I don't believe there is a cookie cutter that fits.

[1] Somewhere in Disciplined Entreprenuership, Bill Autlet, MIT