We started emitting radio waves just over 100 years ago. Initially these were all low frequency that were mostly blocked/bounced by the Ionosphere. Nowadays we mostly use very high frequencies and whilst they go straight through the Ionosphere like it's not there, we use a tiny fraction of the power we used to use for many of these. Yes there are still many high power broadcasts but fewer and fewer as time passes. Eventually we will move on to everything being cabled/fibre except for local very low power (WiFi, Bluetooth) and eventually to Ansible comms or something similarly weird. So, I would imagine a lower limit of 100 years and an upper of maybe 500 years for the bubble thickness. It doesn't means the aliens died, they just stopped using broadcast high power transmissions in favour of something less polluting and more efficient. It could even be that once we discover the next comms revolution, the aliens are all there and willing to chat with the noobies from Sol3.