I got a ECM Sync flow control about 11 months ago. I have it plumbed in with a water filter system. I understand that I shouldn't have to descale very often, but is there a place to check for any buildup?

Where does most scale buildup? Mushroom?

From the FAQ: Checking an E61 Espresso Machine for Scale. Scale buildup on the steam boiler's water level sensor is easy to check; if you want to be extra thorough, remove one of the steam boiler fittings and use a endoscope camera that connects to your computer/smartphone.

One of the easiest places to look for evidence of scale would be to examine the top of the aerator screen at the end of the hot water spigot wand to see if there is any accumulation of precipitates.

Preheating cups before shots with water from the hot water spigot is a good practice to evacuate scale or anything else that might gather at the bottom of the steam boiler. Checking the aerator might indicate if you're heading into troubled waters.

I know this is a tough question, but how long should it take to build up scale if I'm using a good quality inline water filter?

I have no idea. It depends on the hardness of the water, how effective the water softener is, how much you steam, etc. I descale once a year, but I make very few milk drinks and the water in our city is fairly soft. If you want a more specific answer, I suggest starting a thread in the Water forum.