I used a generous 1/2 an oz in a wit and it was clearly the dominant flavour in the beer. It wasn't bad and if you like chamomile tea, you would probably enjoy it but it was definitely not subtle. I also had coriander and orange peel in the beer and they were lost behind the chamomile. Mine was bulk flowers, not tea bags.
I would assume that the general profile of the beer would be a factor. If it was a light, neutral beer less may be enough. In your Belgian, if there are strong phenols, you may need more for it to get noticed.
I'd start at 1 oz and go from there.