I have my own personal spreadsheet. I figure 1 shard per 3 light/dark mission, and a 40% drop in the Cantina (9.6 buying 1 refill).
It takes 385 shards to 7 star a character so a character with only one "force" mission will take 385 days to 7 star.
Cantina shipments I figure 5.3 or 5.6/5.8 shards per day based on the energy cost (buying 1 refill).
GW I assume I finish every day, so 15 shards a day takes 24 days even to 7 star.
Arena shipments I finish top 50 so I get 10.625 shards per day, however if you can get top 100 the math is simple at 38.5 days to seven star.
You need to start the long farm goals of say Anakin or Barriss ASAP....but you should focus on getting your arena 5 in max gear first...its hard to balance but I would say pick a few (3-5) heroes and always farm them and dump the rest into gear. I buy 2 refills a day.