I don't believe there is a standard retainer for all professional advice.
Your success is not based upon more activity - i.e. more clients paying similar retainers - it has to be about more productivity. Otherwise you are going to be working ridiculous hours to keep up with the workloads.
The first step to greater productivity for professional advisory firms should never be hiring more people. The first step should be improving your pricing.
This is my third webinar before Christmas, and it's about the role that professional and ethical pricing plays WHEN building a principal advice model - i.e. advice that positions your firm as the client's principal adviser whilst ensuring there is NO product or hourly conflicts associated with the advice.
I look forward to sharing this webinar with you.