You should only declare a specific time for manifestation as the Holy Spirit leads you. Spend time with God. If the Holy Spirit said 7 days, 2 months, or 2 hours for a thing to come to pass, that’s what you decree. If He doesn’t give you a specific date/time, just believe in the manifestation of your promise at the God-appointed time.

The devil will tempt you to back off of your belief and confession about the time God gave you. He’ll let you know how unlikely it will manifest in that short period of time. But if you believe God said it, declare it regardless of how it looks.

The Holy Spirit knows the level of your faith. He knows if you need 6 weeks, 7 months, or 2 years to believe for the manifestation of a certain promise. Don’t try to think you’re at another level when you’re not. Don’t think you’re at a 2 week manifestation level when He’s told you you’re at a 2 year level. That only leads to self-inflicted disappointment.

The Holy Spirit knows the level of your faith.
He knows what you need so that He can help you decree things that are within the framework of what God can do for you because of where you are in your belief.

Spend time with God about a certain promise. If He gives you a time when that thing will manifest,  declare it, but if He doesn’t, continue to believe that you’ve received it until it manifests.

Ecclesiastes 3:1 There is a time for everything, and a season for every activity under the heavens.