Twitter has a little over 800 million shares outstanding.
9.2% of that is 73.6 Million Shares.
On average, 19.2 million shares are traded.
If he were being extremely aggressive, then he could have accomplished this in 4 days, but he likely did it over a longer period of time so as to not drive the price up.
You have to remember, there aren't just people buying and selling shares, you have various funds, indices, and institutions tradings these as well as day traders, swing traders, and market makers.
Twitter is a very liquid stock.