A majority of the game's playtime is spent on the 5th level. If the scoreboard tells you how many seconds it took people to complete this game, then it may take a solid 2-3 hours with no premium weapons.
I agree with tymerum, the scaling of the prices are a bit wonky. In late game, the cost of upgrades are kinda reversed... where upgrading oil companies is the cheapest, and upgrading newspaper delivery is the most expensive.
I agree with the difficulty being too low. Never had to use the explosives upgrade, and never got hit during the 40 days... and I'm using a touchpad to play on top of that. I did primarily use the high accuracy weapons though, and then switched one of them out for a grenade launcher towards the end.
It helped me a LOT to drop a bit below the pipe before flapping up through the entrance, because coming through the pipe too high greatly increased my risk of flapping too soon/late due to how quickly the bird's fall accelerates.