This is only my impression. I do not have any statistics to back up.
What is the accuracy rate of the forum signatures? And by accuracy I mean the CTR or Click-Through-Rate.
how much visitors can you expect /week?
Depends on what the advertised service is and where the sig is posted. Assuming you are refering to sig campaigns, my guess is that CTR is high in the first couple of weeks and gradually falls afterwards. I believe a majority of the members here would have seen a version of the sig at least once in the first month, but they might not have clicked on it. The effectiveness of the sig ads shouldn't be measured by just the CTR, because sig ads are exposure for the site. Some services requires longer exposure to gain trust.
But i just want an estimate, for example for my account a Member , how much visitors can I expect for example?
If it is just you posting (1 member account posting 20 posts a week), a handful a week would be an optimistic estimate.