So with the expansion of the cap to 80 there is now a credit shortage for all player levels. At cap level 70 provided you didn't buy shards you had enough credits to level and star all you charactors.
You get 15 GW shards, 10+ arena shards, 5 cantina ship and 7-10 cantina mode shards per day for a total of 40 shards per day and 280 per week.
It cost about 4.5 million to max a charactor(plus abilities) and 330 shards. Total waste of income in the game is about 4.5 million. Looking at the math here you could level and star your charactors and over time provided you got less than 50 extra shards per week eventually it would level out.
Now with 6.5-7 million require to max 330 shards you are short. It now takes 10 daysish to earn your 6.5-7 million credits. This means any shards more than 33 a day can never be fully levelled.
So this means you should sacrifice 50 shards a week to the shard shop to speed the farming of other characters. If you buy chrome and do lots of hard nodes this number becomes higher.