Musing received its largest delegation (997,273.813 SP) from the misterdelegation account, on October 26, 2018.
The misterdelegation account is owned by Steemit Inc., and as the name implies, it is used for large delegations, principally to other dApps, along with accounts belonging to crypto markets, and spam fighting entities.
Musing is currently receiving a total of seven delegations, the next largest from roadscape (75,646.049), followed by yunoyang (1,010.183) and project7 (1,009.666).
MIsterdelegation currently delegates to 21 accounts, including:
binance-hot, bittrex,, blockbrothers, busy.pay, dsound, dtube, esteemapp, fundition, mack-bot, oracle-d, poloniex, sndbox, spaminator, steemcleaners, steemhunt, steemit-jp, steemitboard, steempress-io, tasteem, trendings-grace, utopian-io.
Three of those accounts have over 2 million SP in delegations from misterdelegation—dtube, oracle-d and spaminator—while a fourth—utopian-io—receives just under 2 million in SP.
All told, misterdelegation has nearly 18.5 million SP delegated of its 18.86 million SP.
How long the delegation might last is not something that's been made public as far as I've seen. Most of the delegations from the misterdelegation account came during 2018 (or they could have been reduced or enlarged at that time, too).
Why are you interested in where musing got its 1 million sp delegation from? That should not be our worry for as long musing is functional and growing bigger and stronger. Though if you still want to know where musing got its delegation from, you can contact the CEO of musing: only him can answer this your question accurately