It all depends which 'mode' the car is in.
There are effectively 5 main modes - EV, Hybrid, Hold, Charge, and what I think will be called Sport - called 'GTE' mode in the VWs.
EV and Hybrid are as you'd expect. Hybrid will try and and maximise efficiency. Hold keeps the battery at the level of charge it's got in it- for example, if you are going on a long journey, you can use Hold to keep some charge in the battery for the city driving when you get near your destination. Charge is self-explanatory. Sport will keep the petrol motor on, and will always deliver full power, and will also charge the battery.
The battery in all modes is kept over a certain percentage- I would imagine roughly 15%, given a 10% buffer for the health of the Li-Ion cells, and 5% buffer to use to torque fill. If you're at that 15%, and floor it, then you'll dip below that 15% briefly, but once you're off the accelerator the car will charge the battery back up to 15%.
If you keep the car charged up each night, you improve your fuel economy and you can also do some driving in EV mode. If you leave it in hybrid mode the entire time and never charge up, then you'll be unable to go very far at all in EV mode, and the car effectively runs much like a Prius.
The new Octy vRS is available with the 245PS version of this powertrain BTW