Does bitcoin exchange rate varies from time to time?
Yes dude ,Bitcoin price is changing every minute even in a second just like the fiat currency.
If so, when is the best time to convert bitcoin into money?
That depends , for me if i get a total of 30%percent of my total invesment i will convert it to cash.
Is it necessary to just save all the bitcoin and trade or use it after several years?
Yes dude if you considered Bitcoin as a long term investment.