One of the first questions I am always asked by potential clients is "How long will this take?" It is a difficult question to answer because if I say too short, I run the risk of not giving myself enough time to build the system and having to work for free for part of it. Or I can say too long making my price unrealistic.
Some people also expect you to be able to build a system within 3 days and pass an audit no problem. Funnily enough these are also the same people that don't want to have any involvement in it and generally cause headaches. I tend to turn down those jobs.
In order to build an effective Management System I would suggest at a bare minimum without the assistance of a consultant 1-2 days a week for 2-3 months. This would take into account documenting your system, implementation, auditing, corrective actions and review. With the assistance of a consultant you can shorten this as the learning curve is not as steep. This is assuming you are a small to medium business with around 5-25 employees.
The main variable however is how complex your business is. For example you may be a large business with a lot of people, however your processes are very simple as opposed to a small business with complex processes. As an example a cleaning business with 250 employees would have less complicated systems than a microchip manufacturer employing 10 people.
These days I err on the side of caution. I would rather say more time than I think it needed and come in under budget than get halfway through and realise there is no way I will be able to finish the system with the amount of time I've given myself. I've done both and I know which one feels better.