I found (twice)that walking from St Jean to Santiago in 33 days was perfect for me, walking at my own pace, some days 30km some days just 5 and one complete day off in the middle.
I booked my flight home before I started and set it for 36 days after my start date.
This gave me some pleasant options for the end. It meant that I had days sapre if I had to rest due to injury, it gave me time in Santiago to just sit in the square and see friends id lost come in behind me (very special moments) and it also gave the opportunity to catch a ride to finisterre if I wanted to.
Of course everyone is different but many people manage 33 days as a nice average.
These days, leaving tickets until the last minute is very expensive, especially europe's low fare airlines, as an example a flight with Ryanair this thursday one way from Santiago to London costs $220 US. Most international airlines will charge you in the region of $200 to change a ticket you already have but then you would normally pay the difference in fare from when you first purchased a ticket.
Its a tough decision, but ultimately money is best spent enjoying your camino not wasted on tickets changes....over estimate your trip, there are always things to do if you have 3 days to kill...in fact last time I was in Santiago for three days it wasnt until my third day, and 3rd mass that they swung the inscense burner so was glad of that!
As for the crowds, July and August will be heaving with pilgrims but reports are that numbers are high even now. As for bugs, twice ive walked and never had bed bugs, only bugs that caused problems was one single day in the height of summer that dive-bombed me all day long, annoying more than anything.