I have a Sparkasse bank account(Germany) And I have a MasterCard too. And I am able to link both of them on the website, and I'm also able to transfer money from my bank account into my credit card(MasterCard), if I do that, how long would it take for Sparkasse(Or whoever handles this transaction between me and myself.) to transfer the money?
In a nutshell, as long as they (Sparkasse) choose to.
I work with banks where it happens the moment I submit the transaction (so the next screen already shows the new totals), and I work with banks that make it take 3 days.
In the past, Sparkasse and Raifeissenkassen were especially famous to take a looong time ('Wir nehmen mehr als Geld und Zinsen...' - they supposedly work with the money inbetween, as it is gone from the source account but not arrived in the target account yet); that might have changed (or not).
Probably Sparkasse has a statement in their fineprint on how long they make it take. I would expect one business day in today's environment, but I didn't look it up.