With a 7kW charger I would expect 50% charge to be added in about 6 hours active charging. If you are using "Smart Charging" such as Intelligent Octopus the charging scheddule may (most probably will) consist of several shorter time slots spread out between the time that you plug in and the requested availability time but the totatl time actively charging would be the same. For Intelligent Octopus you could look at the detail of the initial schedule (it usually changes during the night as charging progresses, and even before it starts) and work out the total time planned, which is usually a bit more than "required added percentage" x 6/50 hours.
If the car is adding significantly less than 50% charge per 6 hours then something is likely to be limiting the charging rate. This could be that the car is set to use a reduced charge rate or the "charger" is limiting it for some reason.