How can Icalculate number of days worked each week based on data entered in another field?

You would need to summarize qualifying days (days that have a conversion) up to some parent record, maybe a weeks table, or a months table.

Depending on the level of details you need and the historical reporting needed you might need some joined tables.

What are some of the end results you are looking to do with that "# of Days Worked"?
What reports?  What time frame?  Historical needed, or just a rolling total?