Depends on what your question means--you want to do ecological restoration of a former farmland, rangelands, wetlands or riparian areas ,and bring it back to its original conditions--and then for what purpose, who's land and with what funding, and in what State, and then within which ecosystem?

Speaking from a central California perspective, there are dozens of layers of knowledge you must know, and my work currently is in serpentine grasslands at Kite Hill in San Mateo County, and also studies of the Arana Gulch Coastal Prairie grassland in Santa Cruz County. Even though these grasslands are located only 50 miles apart, and the methods to restore are very similar, and the species and ecotypes of natives are very different.

For example, in the serpentine the main grass is the annual Vulpia, where none is evident in the Arana Gulch meadow. And the main grasses at Arana Gulch are Danthonia and Bromus where those are rare at Kite Hill.

The important issue, is both areas have had a very different history with the European people who brought their grazing animals and changed the soil nutrient and organic matter layers of the soil, and when exterminating the natives, the soil fungi and bacteria that were symbiotic with the native plants, have been gone for 100-150 years.

The other issue is dormant native seeds still in the soil. I have been lucky at Kite Hill, to find dormant native seeds still in the soil underneath the weeds, so once we get the weed out, the natives sprout up where they last grew, I estimated somewhere between 150 and 250 years ago, at the rate of 5 per square inch.

Some native species will sprout up when you can produce less than 50% weed cover, and others wait until you get an area cleared 99% of the weeds gone. You can read how we did that on 74 acres south of Santa Cruz, went from 99% weed-covered to 95% native cover in only 8 years at Article Releasing the Native Seedbank: An Innovative Approach to Res...

There are also other reports on Research gate like Preprint Arana-Gulch-Box-test-for-seedlings

ETC.

If you make your question much more specific, where and the goals, you will can get better answers. Picture from my Kite Hill project. The goal of any project, should be 95% native cover within 5 years or less. You can see 100% native cover in six months north of Reno in a cheatgrass area in 8-10 inches of rainfall, that remained 95% native cover 25 years later at https://www.ecoseeds.com/greatbasin.html