Is there a fixed period of time for people who are within the miqat (place of assuming ihram [ritual consecration]), in Jeddah for example, during which they should not enter Al-Masjid Al-Haraam to pray therein except in a state of ihram? What are the conditions for visiting Al-Masjid Al-Haraam in terms of time-frame?

All perfect praise be to Allah, the Lord of the Worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

For those who dwell within the miqat, they are entitled to visit Al-Masjid Al-Haraam and pray therein at any time they wish, even if they were not in a state of ihram.

When they wish to perform Hajj or ‘Umrah, they should assume ihram from their homes, and it is not incumbent on them to go to the miqat. This is because the Prophet, sallallaahu ‘alayhi wa sallam, said:

So, these (above mentioned) are the mawaqeet (pl. of miqat) for all those living at those places and for those who come through those places with the intention of performing Hajj and ‘Umrah, and whoever lives within these places should assume ihram from his dwelling place, and, similarly, the people of Makkah can assume ihram from Makkah. [Al-Bukhari and Muslim]

This means that whoever resides within the miqat should assume ihram from his house in order to perform Hajj or ‘Umrah.

There are no time-related conditions regarding visiting Al-Masjid Al-Haraam. One may visit it at anytime of the year except if there is a (religious) impediment such as menstrual bleeding in case of women.

Allah Knows best.