Having spent 2.5 years with "BIG 4-ONE" on Manager and then 2 years with UK's leading bank (overall 15 years experience) I applied for Senior Manager at "BIG-4 TWO" and was offered manager (Top Band) with the promises to make SM in 6 months!!
After joining they put me into a role "under another manager" who was with "BIG4-TWO" for over a decade and was also contending for SM promotion for the second time!! No issue that another manager was also contending but the issue was how the heck I am going to make SM if I will be functionally reporting into another "Manager" at client site.
My concerns were acknowledged by my onsite line manager (director) who clearly told me that in that role I am not going to make SM "ever"!! I felt betrayed by "BIG-4-TWO" and nothing was done by "BIG4-TWO" to keep their promises which were the basis why I accepted their offer and joined at Manager level.
It's didn't take me much to sum up that I have made the worse mistake of my career to date that I believed "BIG4-TWO". I resigned within my first month by giving them 1 month notice ( I had to make quick decision considering after first 3 months my notice period was increasing to 3 months and it would have been difficult to get out of "BIG4-TWO" that easily!)
I don't regret leaving "BIG-4 TWO", I secured a contract in a month paying me 1.5 times of what I was getting at "BIG4-TWO" and today I manage > £10 million projects and programmes for a leading UK Bank!!
Summary, you will never make manager in first 12 months. if you think you are at manager level then don't compromise any level below that. As someone said 30-36 months is the realistic time to get promotion and that's was told to me by the director I was reporting into. I clearly undersold myself and had to leave in a month due to frustration...you may not want to do the same....
Having "BIG4-2" on my CV only for couple of months didn't really hurt my career but I have recently learnt not to risk my career any further.
I have 3.5 years old daughter and it's really amazing feeling to see them grow. Having worked in Consulting for many years I will definitely never go back into consulting due to work life balance issue.
PS: If you are not convinced then you may want to replace BIG4-TWO with Deloitte and then try to feel how frustrating it can be for you if you land up in similar situation ;-)