My view is that you cannot impose a timescale on the client as it is the client's responsibility, what you should do is to give an idea of the priority of the actions you are proposing.
The problem with timescales is that they will vary dependant on who you are dealing with. For example the action to install a fire alarm system, for a small company where the owner MD etc is all rolled into one three months may be acceptable, however for a a site which is part of a major company by the time the person in charge of the site has obtained three quotations, made the business case, submitted that case, Head Office has examined it, the board of directors has reviewed it, decided whether it requires emergency funding or is a case for capital expenditure and from which financial year it will come out of you will be lucky to get a decision within 6 months let alone get it installed.
I tend to use an Immediate, High, Medium and Low priority scale. Immediate are actions which need to be carried out preferably before I leave the site, High are the actions that need to done first, then move onto the Medium priorities etc. but it is up to the client to decide what is to be done and when and then, if necessary, justify it to the enforcing authority.
The only people who can impose a timescale are the enforcing authorities via their enforcement notices.