There is no generic formula for this and wall thickness is only one of many variables. All of the following and more are other variables that can greatly change the cycle time:
number of cavities in the mould
type of material
any slides, core pulls, or unscrewing mechanisms will all add time
robot for part or runner removal will add time
geometry of the part
design of the mould
type of injection moulding machine that is running it
runner type
gate type
