Comment convertir mAh en minutes/heures ?

It's not that simple to convert... and you can't turn turnips into cucumbers:
- 1800 mAh: your battery provides 1800 mA for 1 hour at its operating voltage, or 3600 mA for 30 minutes, or 900 mA for 2 hours,
- if you know your PC's energy consumption (in Watts or mW, 1W=1000mW) and its operating voltage, with 1mW = 1mAmpere x 1 Volt, you can calculate its current requirement in mA and deduce the operating time.

All of this is nice in theory, but it only works in theory and the battery capacity is dropping rapidly!

The simplest thing is to read the PC's manual...