Very close to all of the time, I only plug in when playing with others, recording or if I want to use effects.
This subject has come up before and I noticed a lot of people stated that they couldn't hear themselves properly and developed a bad habit of playing too hard. I wonder how much of that is due to hearing loss, and because of noisy environments.
I find it is easy to hear myself playing unplugged, I have practiced this way since almost from the start. At the very beginning I used a little Yamaha Budokan 50W guitar amp, then I used a Park B25 MkII 25W bass amp.
When I started to get more serious about playing, I used an amp less and less (when practicing), my feeling is when it's just you and the strings, with no setting, eq, gain stages etc. There is less opportunity for excuses, you can focus on your "real" sound, learn to manipulate the strings to produce different timbres. Basically if a note is flubbed, out of time, uneven volume etc. It's you. And that is exactly what I like about it, you play crap and you have to say well I played crap today, and not blame any piece of equipment.
Early on I read an interview with Ron Carter talking about how players these days rely on equipment and don't take the time to develop their sound from the bass and strings themselves.
Although he was referring to double bass, I see no reason why that idea couldn't be applied to bass guitar. A lot of other instrumentalists work on tone production so for me it's a little bit about borrowing that concept too.