Hello Ninah - sorry for the delay.
This is a very tricky area - the old thorn of rejection.
First, it goes without saying that we writers have to put up with it.
First from agents. Then from publishers telling our agents that while they love our latest work 'it doesn't fit the list'. Then when our work is published no-one buys them. Or the bookshops won't put the in BOGOF.
Then some fucker on t'internet will say 'yawn, Litchick's latest is crap...don't bother.'
So the one thing we know will happen is that we will have to deal with rejection.
How much each person can take and how persistent each writer can be in the face of it, is personal, I feel. Some writers are very raw and find it makes them depressed. Others, like me, let it roll off them. The former probably are better writers, the later are porbably more successful and /or happy in their life.
I know writers who have been unable to cope after one or two rejections. They gave up.
I also know writers who had several books that never saw the light of day and are now very successful.
As for each specific project, and how much you should persist with it...that is a different and very good question.
I would say that after ten pro forma rejections, you need to take another look at your sub package. Could it be better?
Then try another ten.
After twenty, if I had no feed back whatsoever, I think I might question the project itself ( as opposed to your skill as a writer).