Hi Jalmari, great question. might be the million dollar question...
so here are my 2cts:
The level of TRUST between colleagues in the organization is the biggest differentiator in speed of implementation. I trumps size and history of the organization.
what I mean is: if the trust between employees is high the implementation of holacracy is much easier then in the situation where there is a low level of trust between employees.
With very high level of trust between employees I have experienced 10-50 people companies get to a basic comfort level with holacracy in about 2+ months. larger companies and lower levels of trust can easily get you to 6-12 month range.