I think it would tale 3 to five years to break even......just guessing
It could take forever to break even. It all depends on how much the conversion costs, price of fuel, price of the replacement fuel, how much you drive in a year, etc.
I can convert a carb'd V8 (mild, under 400hp) to propane for about $700 or less. Thats including 2 forklift tanks, so I dont run out somewhere and get stranded. But I can get the parts alot cheaper then most people because of the career I'm in. I drive about 20k miles a year. I would think that I could break even in under 3-5 years.
If someone were to pay $3500 (like the OP said it cost) and only drove 8-10k miles a year, it may take considerably more then 5 years to see a reward.
A few of the benefits you would see are:
Longer between oil changes (cleaner fuels like LPG and CNG dont dirty the oil as much).
Longer life of spark plugs (LPG engines have smaller plug gaps, so your coil may even last longer doing less work).