Standard models (fast, low cost) use 1 credit per analysis.
Premium models (larger, more capable) use 5 credits and need a paid plan.
Re-opening a document someone already analyzed with the same model is free.
Long documents are analyzed in full, in parts of about 30,000 characters. A document in n parts uses n + 1 analyses' worth of credits (one per part plus one to combine them).
Credits reset every month (paid plans: on your billing date). Unused credits don't roll over.