how much should I charge a client for multiple dogs and walks that go over 30 min?

Depends on so many factors and walking prices in your area.

I charge $20 for a 30 minute walk and $35 for an hour +$5 for each additional dog

so two dogs and 45 minutes I would charge $30 perhaps. That said, it depends on your area. For repeat clients and easy dogs I tend to make discounts.