Dawnmc,
I started double bass end of April 2009, I was in between grade 4 and 5 on violin.
With minimal practice (30 minutes 3 times a week I'd say) I got to grade 5 standard by December.
I stopped practicing then and reached about grade 6 standard by April 2010.
If I did practice I would be at grade 8 now and that's practicing 'normally' ie: 30 minutes 5 days a week, unlike with the violin I practice 1 to 2 hours a day 7 days a week and I am 'still' not grade 8
If you cannot practice AT ALL, you will still learn/progress at least initially up to grade 6 like me, then I think there's a chance you will stay at grade 6 capability with no practice, but I am just guessing based on my experience.....
Orchestral parts it depends really, at my level I can play a good half of the repertoire reliably, 3/4 not so reliably but no way I can play the whole lot. Most double bass bits in orchestra are very simple but I'd say 1/4 of it is fast passages where you do need the skill to play them well.
So it also depends on what stuff you play in that orchestra, you could ask to see a sample of their typical music...
At present there is NO WAY I would be happy being the 'only' bass player in my orchestra, I could not keep the 'section going' on my own as I cannot play all of it, I fake the stuff I cannot play, so I need another bass player next to me to 'cover up' where I cannot come in, especially if it's a bit where it's the double bass leading
In the first 6 months it's a real physical challenge to play the bass, sore fingers (so sore you can cry), lots of stamina needed, and aching muscle like you've done a gym workout, but it will soon go with regular sessions....