It doesn't take any longer than the MEng route. Just your 1-year MSc needs to be accredited/recognised as part of the further learning of whichever Institution you are registered with. It takes a minimum of 4 years work experience (or MPDS as it known officially) + educational requirements. Let's consider two possible routes:
1. X doing 3 years BEng, then working 4 years in an accredited scheme with a company, then doing 1 year MSc (or doing MSc in between his years at the Company). So it takes X 8 years to be a CEng (minimum).
2. Y doing 4 years MEng, then working 4 years in an accredited scheme with a company. So it takes Y 8 years to be a CEng (minimum).
No difference except that they are two separate routes reaching the same destination at the same time.
And if you are on a 1-year placement that is accredited as well, that is you're on MPDS, then after you graduate, it will take only 3 more years (minimum) of accredited work experience to be CEng. Of course, some engineers wait as many as 7-8 years before they apply for Chartership, and they have their reasons.
I am doing my placement with Cummins in Daventry, which is the world's largest diesel engine manufacturer and leader in power generation, with Perkins being their biggest competitor. An excellent company to work for in all respect, except maybe my current location!