it would take a while to reach enough support to get funded. This means more than 2 months.
You are not supposed to have your proposal started while seeking support, this is why you can set the start date to be in the future. That way you can get votes while the proposal is not "running".
That's what I usually do, I know it takes about a week to 10 days for me to get enough support, so I set the start date 10 days in the future, so that when the proposal starts I am over the return proposal already and I don't lose payouts. And the number of days is end date - start date so that would be fine.
On your other thoughts:
if you are not known, I think it makes a lot of sense to make short proposals for 60 days, then another one to grow your reputation and increase the chances of getting to funding (I know stakeholders feel more comfortable funding 60 days proposals than 360 days ones)
Can start date be changed too? Like earlier if enough vote and later if not enough?
I think that can allow for the system to be games. If the total you are seeking is 100 HBD, at 1 HBD/day, with a start date of a month away, and you get enough support on day 1, you can just move the start earlier and get an extra 30 HBD. Now its up the the voters to unvote if they notice irregularities, but whats to say everyone is keeping an active eye on it? I think this is a no.
Moving it later on the other hand, that I do support.
But what if by changing the start date, the end date is also shifted accordingly?
Well then you'd need to allow moving start end end date, otherwise you're just reducing your rewards, and at this point you got the same problem with "infinite proposals being there forever"