Just yesterday I was ready to buy enough miles to redeem them for a business class ticket. But an uncertainty held me up. Today, the offer seems gone! Some Google searches seem to imply that United runs bonus mile specials now and then. But how frequently? I only have two months before I fly.
As pointed out by @Gagravarr in comments, take a look to posts on LoyaltyLobby in the category United Airlines Milage Plus.
These sales happened almost monthly (though many of those targeted) recently, but this being a volatile marketing instrument, you should not completely rely on another such sale coming up.
The sale you refer to apparantly was this one, apparently targeted and offering miles for sale at up to 75% bones w.r.t. the usual rate.
Since then (until today, 20 June 2016) there have been the following sales going on:
- 50% bonus from 29-31 March 2016
- up to 100% bonus from 11-25 April 2016 (targeted)
- up to 100% bonus from 09-23 May 2016(targeted)
- up to 100% bonus from 24-27 May 2016
- up to 60% bonus from 06-13 June 2016 (targeted)
Do note, as pointed out on Loyaltylobby, that you can only
Purchase up to 150,000 miles per account per calendar year.