Above 500 characters at a time, you will need to pay. Moreover, the service is best suited for writers of Russian, people who already have a working understanding of the structure and style of the language. If you're just inserting the nonsensical dregs that pop out of Google Translate or some other algorithm, Orfogrammka's results will be comparatively flawed; keep in mind that it's not a translator, but rather a grammar checker. If someone is sending you text in Russian, then this service will do little beyond helping you correct any errors that may be present.
It does its job for people who wish to perfect their writing, but, alas, it is not a shortcut to understanding written Russian - or being understood, for that matter.