When there is an external bleeding from a cut when shoud i worry that it's taking too long to stop i mean after how long?

Bleeding Cut: this all depends on where and how deep the cut is on your body. Certainly any cut that is actively pumping out blood should be seen immediately. Deep cuts, more than a paper cut ought to be seen if they gap or bleed steadily. Almost any cuts to the face that don't close and stop spontaneously, scalp wounds. Almost any cut larger than your little finger nail will need repair and heal faster.

Depends: How deep is the "cut", where is it? Do you take aspirin, as it can prolong bleeding. Best way to stop bleeding is compression with a sterile bandage. If the "cut" is wide it may need suturing before it stops. Simple lacerations will stop within a few minutes of cleaning the wound with running cold water & applying a bandage. If you have Sickle Cell disease get to the hospital ER NOW.