Indeed, not as simple as it may sound; the unit of measurement could be %, distance or surface area, depending on context. Attached two articles for your consideration.
Article What Drives Conversion of Tropical Forest in Carrasco Provin...
Article Forecasting the pattern and pace of Fagus forest expansion i...
Dear Mikwa, it might be helpful to perform a simple NDVI using Landsat Images for a preliminary analysis on deforestation. As you may know that NDVI is a standardized index preferred for monitoring vegetation. You can go through the links which are helpful in computing NDVI. I'm also working on a similar project although not entirely on deforestation but more on an urban setting. NDVI change detection was helpful for me to check the greenness of a particular region for a certain time period.