Tanto tiempo sin verte means “Long time no see”.
This is an informal greeting that you can use at any time with people you already know.
Another version is, ¡Cuánto tiempo sin verte! It means exactly the same.
This time both “Tanto” & “Cuánto” refer to amount of time.
Sometimes we just use a short version, “Tanto tiempo…”
Examples,
Hello Sebastián. How are you? Long time no see! Hola Sebastián ¿Cómo estás? ¡Tanto tiempo sin verte!
Hello Sebastián. How are you? Long time no see! Hola Sebastián ¿Cómo estás? ¡tanto tiempo!
If you want to use it in a formal way you need to change the conjugation of the verb to formal you “Usted“.
Hello Angelica. How are you doing? Long time no see! Hola Angelica ¿Cómo está? ¡Tanto tiempo sin verla!