How many courses over summer is too much?

I’m going to take two 6 week long philosophy courses over summer, but I was wondering if anyone could share their experience with taking more than two courses over the summer. Was it difficult? What did you take? etc. I took Calculus 1, General Bio 2/General Bio 2 lab one summer (9 credits) and I found the combination not difficult but then again they were I think 10 or 12 weeks long. I haven’t the slightest clue about how philosophy courses work.