There is no character cap on the usage of itemprop description. The whole point of Schema is to provide correct additional information. By introducing a cap they would be defeating this objective. What they recommend is merely a recommedation and in otherwords they are informing you not to use words that don't describe what the item is about.
If you have an article about Car Manufacturing in 1990, talks about the company ford and the inventor then your description should be something like:
"A in depth look at Ford Car Manufacturing in the 1990s and the inventor William ford"
A bad description would be something like
"Ford was known for manufacturing the best cars in the 1990's, find out about how William ford made the best cars"
Large sentences generally have lots of
. ideally you should only ever have one full stop and if possible avoid commas and then you can't go wrong.