How to calculate how long the battery lasts?

It depends on what. If 2600 mAh are connected in series, and the tape is only a meter (14.4 W), then for two hours.

You think so:

1. Multiply the voltage of the can by its capacity. Or the voltage of a chain of cans connected in series (each approximately 3.7 volts). Take the capacity in Ah *, that is, 1 Ah * is 1000 mA * h. Get a watt * hour.
2. Divide this number by the load power and get how long it will work until the discharge to full zero!