Hi Afroz,
All the questions you asked were answered before, but I'll summarize it for you.
Maintaining it for 6 months is not a mandatory thing. It is advisable though. There won't be any questions regarding the source of funds if your account has that balance for >= 6 months.
When you get a letter from the bank, post ITA, while submitting your application, that letter just gives the total balance as on that date and probably the 6 month average. Now, CIC will expect you to show the proofs if there is a significant increase in those funds between months, to clarify that you haven't borrowed it from anywhere.
So, it doesn't matter when you deposit your funds as long as you have correct paperwork which states that all of this is your money.
If your dad is gifting you the amount at any point of time (less than 6 months before submitting), you have to get a gift deed saying that it's a gift from your father and you don't have to give it back to anyone.
Hope it's clear. Cheers