It depends on what you want to power with it... The installed batteries have a combined capacity of 108Wh. The factory specifies: 24 seconds at full load, 8 minutes and 42 seconds at half load.
I use this thing for a Synology RS1221+, a larger switch (no PoE) and a couple of Raspberries. According to estimates from the NAS, the UPS should last at least 25min.