You're right about that. Thats why I mentioned what kind of laser I use them on.How much run time you get off any battery will depend on the load applied. so sopark's answer probably wouldn't realistically have much applicable use to you unless you were using the battery in the exact same kind of laser.
For example. I have a battery that will run a 1.2W 445 for a little over 2 hours. Yet that exact same battery will run a 260mW 660 build for well over 5 hours. It all depends on your load.
You really need some kind of reference to get any meaningful information.
Hmm. Then assuming your laser is drawing around 1.3A after driver losses, that puts the capacity of them somewhere around 2000mAh.
I suggest that you only buy brand-name 18650 cells.
I have been building commercial UV lights and lasers for several years,
and have used hundreds of 18650 cells.
I now only use name brand 18650 cells like Panasonic, Samsung or LG cells.
Typical capacities for reasonably-priced cells are 2600-2900 mah.
Internal resistance is a measurement of how much voltage drops when you have a significant load on the cell: