Ah or Ahr is Amps times hours. So if a battery is 1Ah, you can draw 1A from it for one hour if starting from a full charge.

However, batteries are normally rated by drawing enough current to last 10 hours, then multiplying that by 10. The faster you draw current from a battery, the less efficient it is, so a 1Ah battery may only give you 1A for 45 minutes before it is empty, but go 10 hours on 100mA.

So you need to start with an approximate number. How large is the battery? What kind of battery?