I don’t know if there is an exact number of reviews per week that Google would or wouldn’t look at. The key is to make the review process part of your point of sale. This way, you will naturally get reviews. At one review per week, you would have 52 reviews in a year, and I see VERY few industries that have 50+ reviews in total on a listing. What you want to avoid is going six months without reviews and then in a single day getting say 10-15 reviews. That is extremely unnatural unless you are in business for one day a year.
The hardest part about the review process is that you need to get reviews from accounts that have review history. If you have a customer who creates their Google+ profile simply to leave your company one review, I can almost guarantee that it will get filtered.