Yes, that is a rough estimate of the space needed. For optimum performance you will need more free space, though. If there isn't enough disk space for PostgreSQL to move files around efficiently then it will slow things down somewhat. Ideally you should have at least as much free space as the size of your database (although that is not actually required).
The conversion process may take some time for that many hands, but exactly how long will depend very much on your computer.
If you still have the original hand history files you could import from those instead and then you could do it in batches when you're not using your computer for instance. If you do that, disable the housekeeping options in the
Manual Import until the final import - there is no point doing it after each partial import.
PT3 will handle 1M hands just fine (as long as your system is up to it - see the Minimum Requirements in the
FAQ).