How often do I have to change my thermal paste?
Every two years as the paste will dry and be less able to transfer heat from the processor to the heatsink.
How often should I get my PC cleaned?
Anytime between 2 weeks to 2 Months depending on how much you use it.
Why do I need to have my cable managed?
You don't need to have them managed but it make your computer look better if people can see inside of your case when you have guest over the added benefit is that it allows better air flow, lower temperatures for both processor and graphics card and no nightmare of where this plug for this part.
What does thermal paste do?
Thermal paste a better contact between the heatsink and the CPU/GPU so that heat can transfer quicker and easier between each other.