@Sloop John - lots of people here with PHEVs are very happy to chime in with their experience. Might be better to wait on them before guessing.
PHEVs will
not fully deplete the high voltage battery. The min charge state before the vehicle switches into HEV mode is 14%. The battery may continue to dip below this during HEV mode depending on driving conditions, but you will be locked in HEV mode and can't drive far (or fast) without the ICE engine starting up. Can't recall off the top of my head what the absolute min state of charge is before the ICE engine fires up at a stand still (sitting still while running the A/C is a great way to test this) - I guess it's 9% or so.
All of these are the displayed battery charge state. This is different from the
actual battery charge state, which is invariably greater. There is reserve battery capacity that isn't displayed to account for cell degradation over time. An OBD2 reader might get you the
actual state of charge. Regardless, just go by the displayed state of charge.
HV Li-ion batteries DO NOT like to be 100% fully discharged. This is bad bad bad. Recall that there will be some self-discharge while the vehicle is sitting - I have no idea how much but something on the order of 1% a month is my guess at an order of magnitude. The charging logic will do everything it can to ensure that you basically can't discharge the HV battery, from limiting the extent to which you can discharge during operation to maintaining a bit of "reserve" charge, etc. It's not something you really have to think about on a daily basis.
My advice is always...just drive. Do what you can to charge when you can. Don't sweat it if you can't. I wouldn't think twice about hopping in and taking off on a break-neck, 3,000 mile, coast-to-coast, no time to stop and charge, interstate slog. It's not my cup of tea, but based on my long distance driving experience in this car, it can handle it no problem.
All that said, my habit is to kick it into HEV or Auto mode once I'm on the interstate at the beginning of a long, long drive - especially if I know there's no charging at the end. This saves me some charge to use at my destination where I'm more likely to be driving around locally where the EV mode is the most beneficial. If you leave it in EV mode it'll run down to the 14% cutoff and then lock into HEV mode until you can charge.