Why does time matter?
A hijacker can do so much damage to an account, they can permanently ruin it. Which is what they have done to your account.
If you are in the habit of taking long breaks, I recommend you upgrade your accounts into a Jagex Account.
This is because Jagex sends you an email
every single time
there is a login to your account, which is something they don't do with the older style, standard accounts.
The Jagex Account system is very new, it was released for open use in the first week of April this year. You would have been alerted to the hijacker if you were getting the login warning email.
Why didn't Jagex lock the account?
They may have after the first ban was removed, we don't know.
However, if the hijacker had access to your email, it would have been simple to unlock the account.
Protecting the security of an email is the players job, not Jagex'.