1-2 minutes seem a fair allocation to get the plan in action - Valve shut down?, getting "stable"..., and enacting the exit (are you rigging an up line? are you swimming back to a mooring line? are you shooting a bag/smb?) all takes some time to execute. My last choice in an exit strategy is a free ascent from depths I'm usually diving.....
I'll set aside time in my evaluation of gas to cover this.....
If you are using a "small" pony, your only plan on an OOG is ascend now. If your buddy is close and in contact, of course you can swim to him but that is not something a self reliant diver counts on.
if valve shutdown, solving other issues or getting "stable" is needed because you have not drilled with your pony, then you need a BIG pony.
It's a mindset, if we are talking OW you always have a direct path to the surface, using the small pony limits you options but is less of a hassle. The big pony gives you more options, but at a cost in dealing with another big piece of gear. (Yeah its light underwater, its still a big piece of gear).
We have the same issue with guns (another fun topic). Whenever you starting talking about what size is big enough, someone will top you and say you need a bigger size. Does not matter where you start, someone will cite some incident where a drug crazed zombie was only stopped with a .50cal to the brain box. The truth of course is 99% of the time you don't need a gun. The 99% of the time you need a gun, any reasonable size will do. Now we are getting all worked up on solving the 1% of the 1% of the issues, while still doing stupid things like not monitoring gas, not planning dives or maintaining gear.
---------- Post added August 16th, 2013 at 11:10 AM ----------
... I carry a pony when solo over about 40 feet but consider the surface as my redundant gas above that. A pony at shallower depths is more a nice to have rather than a must have thing. That probably makes a difference as to whether it is perceived as heavy and cumbersome.
I came to the same conclusion. Hiking up to the car after a beach dive with full cold water dive gear on really makes you consider how much weight you want to dive with. Thus the pony often stays home when I beach dive and know my max depth is shallow. When I do dive with one of my pony's, I choose the tank based on the conditions. Diving a oil rig with no real bottom would drive me to choose larger tank than hitting the local spring with a 84' bottom.