How to say something “gets old fast”?

In Colombia we would say

Esto se esta poniendo cansón/mamón.

That would be used when initially something is ok but quite fast is not fun anymore and it translates "this is becoming boring"

Another way could be

(ya) me estoy cansando de esto.

that translates "I'm already getting tired of this." and the unpolite way would be

¡me mamé de esto! / #YaNosMamamos / ¡me mamé de esta pendejada! (*)

(*) Use with caution

I think the most common way to say that in Spanish is:

Cansar rápido.

You can find the following definition for the word cansar:

cansar

Del lat. campsāre 'doblar un cabo', 'desviarse del camino', 'virar', y este del gr. κάμψαι kámpsai.

4. tr. Hacer que disminuya el interés de alguien. U. t. c. intr.

That fourth meaning says "To make someone's interest diminish", which is what getting old implies.

An example of usage:

Y sobre la televisión, pues... es algo peligrosa, porque la gente se cansa muy rápido de ver la misma cara.

Tiempo, 28/05/1990 : "Terranova" (Spain).

The example implies that people may be interested in watching a new face on television, but that new face may get old very fast.