I don't know of a way to find how much bitcoins are being used to buy from "mainstream" outlets. I've only used online wallets and they don't have stats on where coins are being used. Even if there is a way, that would not include bitcoins used for buying things directly from the vendor. The variety of items available for buying in bitcoins in stores and physical shops are still not that large so there may still be people using bitcoins in f2f transactions.

Ok, I thought about this, the answer is out there and it's more simple than I first thought.

Why do people make transactions?

(A) To buy goods
(B) To pay for services
(C) To transfer bitcoin to a different wallet

What percentage of transactions in a day are conducted by A, B, or C?

A and B apply to the OP's question, so if you add your assumption for A and B you'll arrive at a % of all daily transactions that can be attributed to payment for goods and services. Then multiply that percentage by the daily transaction volume to get your answer on any given day.

Let's say A + B = 85%.

On March 7th the Bitcoin transaction volume totaled $102M. (https://blockchain.info/charts/trade-volume?timespan=all)

85% * $102M = $86.7M - this is my estimate of the amount of transactions used to buy goods or services conducted in a single day (March 7th).

What do you all think of this approach?