A good rule of thumb is to let your metal sit in the forge for around two hours. During this time, the final shape will be more refined and the surface will have a bit of a satin finish
The steel will also cool down faster and become more malleable (easier to work with). You'll also notice that the finished piece will have much less of a mottled appearance than if you had rushed it out immediately after forging.
how long should i let metal sit in the forge