the assessor has provided these timescales to help your company set out an action plan for the works required, and also to state that in his proffessional opinion these outstanding matters should not be left longer than this.
1-3 months is not unreasonable for a high risk. let me give you an example;
If an inspecting officer where to visit your premises to carry out an audit and he considers the risk too high, he has the power to prohibit the use of the building or even prosecute the responsible person. i do not want to appear as if i am scaremongering that is not my intention, i just want to show that in perspective, 1-3 months is not an unreasonable time frame.
i recommend you speak to the assessor and try to understand why he has indicated these timescales.
Pleas bear in mind if something where to happen at the premises, that was directly related to the assessors'recommendations, and your company chose not to implement them within the recommended timescales. it is likely your company would find it's self in court explaining to a judge why a professionals advice was ignored.
i hope this helps