How long tongue cancer does take to develop fully?

Depends: Depends what you mean by fully developed. A cancer can occur and grow within weeks to months and can spread equally as quickly or some can develop over short period of time. Each cancer is different.

Years, but variable: Most cancers are thought to have long "hidden" development periods in which they are not clinically detectable. Many, but not all, go through "pre-cancerous" stages; most tongue cancers are in this category. I'm not clear if you are asking about time to develop after a particular toxic/carcinogenic exposure, or from some other time point. A new diagnosis today likely means it took years to grow.