Worst stretch of mosquitoes, horseflies, biting flies, etc. I've encountered was in the West Siberian plain, between the Ural Mountains and Lake Baikal. Essentially five weeks where except for large cities, the insects were always present. In that regard different from Yukon Territory, Alaska and parts of outback Australia where I've been in localized places with bad mosquitoes but as continuous repeatedly day after day for multiple weeks.
1. During the day, riding there was typically some air movement so didn't have as much of an issue. Mostly needed to keep a watch out for horseflies that landed and would walk around a bit before biting. So keep a guard and keep swatting before they bite. I had a headnet but didn't wear during the day. Have cycled some with headnet in Australia but found it mostly annoying to do for too long.
2. Worst spots were once one stopped to camp. As shown below the regimen was to (a) don the rain gear to cover surfaces as much as possible (b) use a head net. Once we changed, we would as quickly as possible set up our tents.
We would then jump into the tents and as much as possible get any mosquitoes or flies that happened to get in while setting up. After that, the mosquitoes could cluster under the rainfly but we would be away from them. We pretty much stayed inside tents after that.
In the morning, it would be a reverse procedure. Get everything staged in the tent and put on the raingear and headnets. Jump out as quickly as possible to get the tent down and pack everything up.
Some places we camped it was bad enough, I would wait in going to the bathroom until a few kilometers down the road.
Still got some bites, but not as bad as it could be.
As far as DEET goes, I used it somewhat sparingly in Siberia. Mostly because we traveling for multiple weeks and I was wary of longer-term use. It did seem to help when dashed in local places like exposed hands.