How long has it been?

A man gets sent into a land full of monsters and nothing else he is the only human there but his body doesnt seem to age there he spends all day and everyday no stop killing and fighting and honing his skills until one day he comes back to the real word, only to find out that he has been gone for 5 years, and when he comes back the world he knew is different monsters are in these things called dungeons and his word isnt the only one with humans on it!