Then be ready for some crashes on your PC, you can't mine and do video gaming at the same time, not even watching videos on your PC can work, when you mine the miner will use all available CPU and GPU power
Will it affect my GPU/CPU lifespan? Is it necessary to run the desktop 24/7 without shutting down? Just pretty curious.
What's your electricity rate?
If you pay USD 0.10c kWh or less you can simple let your PC mining 24h, and just turn off the miner when you'll play, simple like that
Forget the idea of Gaming and mining at the same time, and it does't worth to mine only a few hours a day, if you want to go deep on hardware lifespan you'll find that run hardware 24h without stop it's better than turn on and off everyday