Shimon, it’s more about their intention during the actual time of tevilah, on the day they converted. If they intended to completely accept the entire Torah as binding on themselves, with a lifetime commitment to observe it, then the conversion is valid and they are Jewish forever regardless of what they do afterward. However, if someone is not observant shortly after undergoing gerus, people often assume there was no real kabbalas ol (though that may not in fact be true.) The only time it really makes a big difference is regarding the Jewish status of a giyores’s children.
If some time went by before they “went OTD” then no one really knows, except the ger himself, because it’s all about the kabbalas ol when he converted. In most such cases, I would assume the original gerus is valid.
Like Shimon, I don’t agree with Levi that any ger who does OTD must not have been an invalid ger. I’m sure there are people who convert completely intending to be observant for life, and yet something happens later that leads them to fall. Hopefully, b’ezras Hashem, in case of this ger, he will make the right decision and return to Yiddishkeit.