"How long is a piece of string", can't say. We did run a Coleman lantern on petrol as a test and got about 7 nights out of it before all the junk in the petrol clogged up the works, other people say it can take years. The biggest issue for me is the smell of petrol, it smells when you burn it more than naptha and any spill and the smell is there for ages. If I had to I'd run something like an Optimus 111b petrol stove on car fuel as its construction is really simple and easy to keep running. I'd never use it in lanterns or more complex modern stoves. Choice is yours at the end of the day. A fivers worth of Coleman is a lot cooking time really, cheaper than liquid gas.
I know the answer to that,
"how long is a piece of string?"
and the answer you are looking for is "twice the length from the middle to the end" hahahahaha......