Every battlebot runs on a battery. It holds a fixed amount of energy (its capacity), and it slowly recharges over time. Your weapons draw energy out of it. (Driving is free in the arena, so only weapons drain the battery.)
Think of it like a sink with the tap running and the drain open at the same time. The tap is your recharge; the drain is your weapon draw.
If your weapons draw more power than the battery recharges, the battery empties. When it hits zero, your weapons brown out: they cut off until the battery refills. A spinner spins down, a hammer stops swinging, right when you need them.
If your draw is less than or equal to your recharge, the bottom of that fraction is zero or negative, so the answer is unlimited: your weapons never run out.
The Stats panel shows a Sustained run stat. That is exactly this number. If it is short (a few seconds), your weapons will die mid fight. That is the single most common mistake when building a bot.
Open the Power Lab tab and try it: push the weapon draw above the recharge, hold fire, and watch the battery die.