Taking a month, or even a year to complete a mission is no different to taking a week. The only problem would be if a faction had retreated from the system - which could still happen within the one week many missions have.
Why your hurry? Are you more concerned about making credits than about enjoying the game? If so, then that is very sad.
I do not believe i have erred - but without a developer stepping in i suppose its a moot argument. Many items in the BGS could have changed over time in the duration between you picking up a mission and ending it. if everyone could pick-up missions for factions in systems all over the galaxy and 'hold them' for months while they play their next favorite game, or indeed deal with RL emergencies (and i am not intending to sound insensitive to any players real world plights - i do sympathize), and then start dropping off missions for factions that have changed during the interim period it may have unintended or uncontrolled knock on effects to the BGS when buckets that shouldn't be filling up with data start filling up with data from mission that should have been reset ages ago. Such is my point. in any case - i think Fdev have bigger issues right now and i am convinced they aren't stupid. if they could make this happen initially, i think it would have been deployed and it is hasn't. But put it in suggestions as a separate thread and i may well have to eat my words...
More generally though, I just feel that players should not have to land in a station just to take on a job or turn in some bounty claims, exploration information, etc. In real life, many jobs allow working and getting paid from remote. I think some jobs in Elite should be like that too.
Obviously, delivery missions etc. should still require landing. But it would be nice to be able to accept a delivery mission from remote, then fly to the station to pick up the goods.