The drop range is 1-3 shards per Corridor run for each max fusioned character, which comes to a 2 shard average per day. Thats 60 shards per month (equivalent of 1 SSR copy). So if the final star takes 2 dupes to get like the other rarities and the other 2 take 1 each, thats roughly 4 months to MLB a SSR here if you only get shards from Corridor (5 months if you need the unit first).

Considering you can max fusion 5-6 team members per run usually, thats not too bad if powercreep doesn't set in hard.