Learn Spanish (Spain): ¡Cuánto tiempo! ¿Cómo estás?

It is a set phrase that acts as an exclamation. You can think of it as a shortened version of ¡Cuánto tiempo ha pasado! (How much time has passed!) or ¡Cuánto tiempo sin verte! (How much time without seeing you!). In everyday Spanish, dropping the verb here is the most natural way to say 'Long time no see!'.

Why do we use cuánto instead of cuántos here?

Because tiempo (time) in this context is treated as a singular, uncountable noun. We are exclaiming about the amount of time that has passed, not counting individual units like days or years. If you were counting, you would use the plural, such as in ¡Cuántos días! (So many days!).

Would a native speaker in Spain be more likely to say ¿Qué tal? instead of ¿Cómo estás? here?

Both are perfectly natural and often used interchangeably! However, ¿Qué tal? is slightly more casual and extremely common in Spain for catching up. You will often hear native speakers combine the greetings, saying things like ¡Cuánto tiempo! ¿Qué tal todo? or ¿Qué tal estás?.