Always use genuine, OEM one, and replace it after 10-20,000 Kms, but depending. If it gets very dirty early and quickly, or you live at a place where sandstorms are there, then it may require very early replacement.
Have it thoroughly cleaned with pressurized air, after every 02 weeks, and when beyond further cleaning, have it replaced with new one.
Same instructions as mentioned for air filter above.
5,000 Kms, i.e. with every oil change.
Brother no idea about it.
I hope, all above helps. Thank you.