Straight and simple, 5 years of stay with social contributions. 

Work permit (not Blue card) need 60 months with social contribution (i.e. you must have a job and pay tax during these 5 years).

If you can find a job which matches BC requirement, you can change your work permit to BC immediately. But think about the total period you need to wait.

Example:
4 mohths later you find a new job and changed to BC, then you'll need 21 months to have a resident permit, the total time is 24 months+21months = 45 months, so it's worthy.

But, if you have worked 50 months, then find a new job which is eligible to get BC, don't do it as you will need another 21 month to get resident permit, in that case just keep your current work permit and change it directly to resident title after 10 months.