I guess I gotta go to the Steam forum to get an answer about Elite, LOL.
Or you could plot a couple jumps and figure it out yourself?
You don't need to actually make the jump to see how much is consumed.
My carrier capacity is 19,765/25,000
3.79Ly jump uses 6 units tritium = 1.58 units/Ly
499.9Ly jump uses 119t tritium = 0.23 units/Ly
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Your question about decommissioning has several answers on the interweb. I would only trust from someone that recently decommissioned one and can answer the specific questions:
a) After the decommissioning request, how soon can I purchase a new carrier?
b) After the decommissioning request, how soon do I get my credits back?
The Elite Dangerous Wiki says:
When a Fleet Carrier is slated for decommissioning, it enters "Pending Decommissioned" status with a countdown timer notifying the owner how long until the vessel is fully decommissioned. The timer will always expire at 07:00
Universal Galactic Time on the first Thursday after the Fleet Carrier enters Pending Decommissioned status, giving the owner up to one week to settle their vessel's affairs. If the owner voluntarily initiated the decommissioning process, they can still cancel it before the timer expires....
...When a Fleet Carrier finally enters "Decommissioned" status, it is taken over by a recovery crew and dismantled.
This suggests if you make the request on Wednesday it will be inaccessible the next day. This article does not explicitly answer the two questions.