The Laminaria hyperborea kelp forests are being grazed by the green sea urchin Strongylocentrotus droebachiensis along the northern Norwegian coast. It started in the 1960-70s and in the northermost parts the kelp forest is still not recovered, we still have huge sea urchin barren grounds. Studies from kelp harvested areas show that the harvested kelp forest reach normal height and biomass 3-5 years after harvesting (Sivertsen 1991, Christie & al. 1998, Rinde & al. 1992). The time it takes to reach full recovery of associated biodiversity varies with latitude (Rinde & Sjøtun 2005), and epiphytic communities are often not totally re-established when the next harvesting episode starts (Christie & al. 1998). This takes about 7 years in areas with optimal growth conditions.

Measure grazed area: can be done by comparing NDVI for two images, before and after graze.

Estimation of recovery time can be computed based on ideal conditions depending on time required for species to recover.

These two documents can probably help you:

-Monitoring Manual for Grassland, Shrubland and Savanna Ecosystems

Volume I: Quick Start by Jeffrey E. Herrick, Justin W. Van Zee.

Volume II: Design, supplementary methods and interpretation.

Also

Huber-Sannwald, E. and D.A. Pyke. "Establishing native grasses in a big sagebrush dominated site: An intermediate restoration step". RESTORATION ECOLOGY . 2005