I think the part that wreaks havoc with relationships is not so much the amount of free time but the timing of it.
For example, if you start off working on your start-up on the side to check your assumptions, that'll suck up a lot of your common free time.
Then, if you work full time on your start-up, you might find that a lot of your essential networking and meetings happen outside of normal office hours. So if your girlfriend is working office hours you might find that you've got a lot of flexibility, just not when she does. So then you find yourself busy when she's free and free when she's busy. Not a recipe for success.
For example, if you start off working on your start-up on the side to check your assumptions, that'll suck up a lot of your common free time.
Then, if you work full time on your start-up, you might find that a lot of your essential networking and meetings happen outside of normal office hours. So if your girlfriend is working office hours you might find that you've got a lot of flexibility, just not when she does. So then you find yourself busy when she's free and free when she's busy. Not a recipe for success.