The 'standard' CB-IR course requires a minimum 40 hours of IF Flight Instruction and needs no prior IF or IFR experience.
If a student has received prior IF Instruction (by an appropriately qualified Instructor) then up to 15 of these IF instructional hours can be counted towards the CB-IR course.
(NB: As you hold an IR(R) you must already have received at least 10 hours of IF Instruction.)
Additionally, if a student has flown IFR PIC in the past, then up to 15 hours of this can also be counted towards the CB-IR requirements.
To 'accept' these credits towards the CB-IR any ATO is required to conduct a 'course pre-entry' assessment flight and, in any case, they may not accept any, or all, of them at all.
So, theoretically, the minimum flight time required for a CB-IR course could be as low as 10 hours IF Instruction plus assessment flight time.
In reality, probably, significantly more - need to reach Test standard, etc