Insect repellent with 30-40% DEET should protect you adequately. More concetration only length the time of protection. DEET is a pretty strong stuff, avoid getting it into your eyes and mouth. Spray some on your hand and then aplly to you face. I got some spray on my sunglasses, it melted away the top coating!!!
I was unscathed for the first 2 days wearig long sleeves and using a moderate amout of DEET. Then we went to the waterfall. It was OK while you're in the water. But when I got up and walk on the top of the fall, that's when I got massacred! Actually these damages were done by sand flies, not mosquitos.
Also if you plan to swim in Sandoval lake, put on DEET before you get into the water. DEET is not water soluable, you can only wash it off with soap.
Surprisingly, it rains in the Amazon! Often suddenly, and with great vigour. Sometimes you'll have a drizzle the entire day, other times it will be an incredible downpour that lasts only a few minutes (but which turns trails into mud))