Hello kjambari,
8 years is a pretty long time to work for the same company and more so - 06 years in the same position. But it does give you an edge over others in terms of stability' - something which the HR guys always look out for. Ideally I would see an increase in salary of about 8 - 10 % annually. But again, moving from a supervisor position to a managerial position would put you in a totally different pay-scale altogether. You would have to do a bit of research on this, though, because when you are discussiing your promotion and your salary you don't want to be seen out-of-touch with the market.
Visit some job sites - look at the salaries currently being offerred for the position you are looking at. Or some websites like
www.payscale.com might also help. Today, information is just a google away!
You also would have to keep in mind that most of the markets are not in very good shape these days and it might take it a while to recover. So, this might not be a time when you can expect a dramatic hike in your salary, even if you get that promotion. So, my suggestion to you would be only this - Do your homework and tread with caution ! But it's definitely high time you moved to the next position - if not in this company, in some other company !
All the best
Johnny