There is no way to really know what any battery is capable of without actually running it down to the discharge end point at the expected load current.
Only when you have actually done that can you confidently display a meaningful time and amp hours capacity figure.
The obvious problem with that is that the battery will then be flat, and completely unusable for a long time until it recharges. Nobody wants to do that, but there is no other way.
Its like wanting to know what time you can run the mile, without getting out of your chair. Only way to find out for sure is to actually do it.