Try 400k tickets and watch the bonus increase.
There're a lot of better ways to spend 800k 
How long this bonus lasts?
I explained how it works a few post back:
There is no simple answer as it's a ratio of how many tickets you buy and how much you spend in multiply games compared to how much you have claimed in free rolls.
So it will last as long as the ratio is above a specified but undisclosed level. They won't tell us what the ratio is to make it more difficult for anyone to game it.
Basically, the ratio goes up when you play multiply BTC, play for jackpots and buy lottery tickets and goes down when you play a free roll. It will last as long as you play/buy enough to counteract your free roll winnings.