Usually, even with salaried positions, you're taking about a 40 hour week-ish. If it gets a lot more than that, it's bad management, though there will be spikes and lulls.
Most companies with unlimited PTO have some guidelines. Like at mine, once you hit 160 hours (4 weeks) there's another level of approval. So you can guess that four weeks is easy, more gets more scrutiny, but certainly isn't impossible.
Every company is different. These are very reasonable questions to ask your manager. "Hey, I just wanted to make sure I understand expectations about..."