hmm I have a sub 1 min GB10 / db10 team i'm at C3 arena. To be honest I grind around 5 hrs a day in both, I don't really use that much crystals because of com2us constant energy events / crystals and weekly World boss / Arena finishes + ToAN / H to be honest I use about 1000 crystals a month. I take about 1 min including clicking time to refresh and sometimes I'm lazy and look away playing or watching stuff but I don't ever use 5k crystals a month despite being on for hours grinding non stop when I am on. The amount of crystals and energy Com2Us give us in Events / ToA / world boss / Arena is roughly enough to last you the month being F2P given you are able to clear toaH as well as normal. Both give you roughly 1k all together not sure the exact amount about there sounds right adding the 5 stage rewards / stage 80 reward.
Conclusion: F2P can last a month easy with sub 1min teams. I'm not F2p Anymore since I buy devilmon packs but my crystals never drop below 5k. Been that way for afew months I've noticed I've never depleted below.