I dunno about the limit your time tho
I routinely stomp on my 53 to prime the fuel pump every spring when I start it from sleep
And I’m lazy I use the starter to drive it up onto loading trailers
Literally a whole 30 second to a minute non stop drive it on low gear with two batteries connected
My opinion is
Use it till it breaks then fix it
And so far I’m the second owner of my 53 on 12v for the last 5 years and it’s doing strong
Plus!
The previous owner used it commercially to haul gravel and fill
So you know it got abused a lot, they busted the rear leaf springs and decided to sell it
It was not treated gracefully
But my experience is anecdotal your mileage may vary
-s