The largest fire in North America was the Chinchanga Fire in 1950, in northern Alberta. It put enough smoke into the air that it was noticeable in Europe, darkened the sky throughout North America, and dropped temperatures in Washington.
Some parts of the fire were hot enough to change the soil chemistry so that trees couldn't regenerate.
Now looking over the region today it would be extremely difficult to find any sign of the fire. The parts that weren't super heated, recovered naturally with most tree species being well on the way to maturity within a decade, and slower growing species at least on the road to recovery.
The parts of the forest where regeneration wasn't possible, didn't make it impossible for trees to grow. It was simply that the usual methods for recovery were destroyed. The pine cones that needed fire to release seeds were incinerated instead of popped, the roots were destroyed, etc. So seeds had to be brought in by wind, animals, etc.
It's hard to find exact records on how quickly this renewal went, but from what I read in the past these spots were very scrubby with patches of trees, surrounded by ferns, weeds and grasses for over a decade. Within about 20 years this scrubby land was on the road to recovery and looked like a decade old forest.
Now boreal forests have evolved to recover quickly from fires, other types of forests will take longer to recover and colonize the dead areas. Also the most intensive flames only covered small portions of forests, a few hundred meters to a few kilometers. Obviously the larger the area of destruction will slow the recovery time.
The recolonization would start at the edges and work it's way inwards. You'll see plants with lighter seeds that travel in the wind creating green zones dozens and even hundreds of kilometers away from the edge, creating oasis' of life in the landscape and making the growth look extremely blotchy. If there is no radiation or other factors, grasslands will cover up a lot of the damage in a few years, even in the largest wasteland. Small weed like trees and bushes will follow quickly, within a decade all but the largest wastes will see scrub lands. After that it largely depends on what type of trees are growing in the area and how quickly they reach maturity to put out new seeds.