So how long can that box run for? Let's say I'd use 150w?

Calculations are pretty simple:

The 2000 W one comes with two 12V 102 Ah batteries

At 240V, that 150W load uses 150/240 = 0.625 Amperes, but we are drawing this at 24V, so the draw at the batteries is 150/24 =6.25A(1)

The two batteries have 102 Ah worth of energy each, which means that at 6.25 draw, you have 204/6.25=32.64h(2)

Now that is assuming 100% of the batteries energy is given to your load. This is not the case in reality. According the specifications, the DC to AC conversion is about 80% efficient.

This means that in order to deliver 150W of usable power, the inverter has to draw 150/0.8=187.5W

Plugging this into (1) and (2)

204/((150/0.8)/24)=26.112 h

edit: ok, fixed it.

https://www.ellies.co.za/wp-content/uploads/2018/07/FBIT1000-FBIT2000-Instruction-Manual.pdf