2040: a $30k neural interface becomes available.
2042: a $20 budget neural version hits the market but requires the user to watch a 15s ad every 5 minutes.
2041: a $30k neural interface that forces a user to watch a 15s ad every 30 minutes replaces previous version.
Every year after: interval decreases by a minute between ads
Let's be honest, they'd put ads in the pay version too.
We would never force a user to watch a 15-second ad every 5 minutes.
We simply augment their content with a 15-second fully-immersive aesthetic and psychological experience which highlights the quality of our sponsor's product.
If the neural interface can filter other ads, that frequency would be a reduction for many people.
We already live in Harrison Bergeron's world, for anyone forced to, say, watch the NBC broadcast of the Olympics' opening and closing ceremonies. "Don't go having unsanctioned thoughts about that artistry, or forgetting about our commercial sponsors!"