It all depends on you. Generally, I'd say 80-20. 80% open to your significant other, 20% to yourself, give or take.
Realistically, people need challenge or mystery, something to admire. That one of a kind special person in the world. If you open yourself up completely and they know you completely, chances are they can get bored with you sooner or later. Again, that all depends on people and people don't stay the same, we all change with time. Keep your wits and charms about you (that's the other 20% *wink*).