Yes, like in all other disciplines. Longterm networks can be compensated by intelligent research for the right people and added value-argumentation as well as a masterplan by using all internal ressources. Shown by myself in 2010 at Opel and 2017-2019 at VW - and never working with the customer engineering and purchasing departments or having skills in airtake manifolds (Röchling/Opel) or mechatronic/complex switches (ZF/TRW/BCS/VW-group) before. Nevertheless I could achieve the first booked business after 3 months.
The answer is depending on my communication performance within my company and to the customer. It is mandatory, that NO Information gets lost and the transfer to my successor follows educational rules (my training exam supports me) including complete handout with easy structure and open To Do's. From my experience, the Interim Manager does the transfer to his successor more effective, because this process is a key asset for him.
Besides Key-Account-Managerment it makes sense to set up an additional Business Development role, independent from operational daily business - to develop special target customers, new products or regions. Or to support product management, that often has to "fight" in sales to get the acceptance for new products. That Saves time for the role out to keep time advantage for innovative new products. Also the support of a sales re-organization works often faster with an experienced person, that is not part of the organization.
The external consulter develops concepts and solutions and the existing staff/management of the customer has to implement them - often besides their daily business. If the Interim Manager developer solutions, he will ALSO implement them by himself in the existing processes. Sometimes the Interim Manager has to implement the solutions of the consulting companies.
At least for 3 months to get really a benefit for the customer. Even when the onboarding process - compared with permanent employees - needs less time for the Interim Manager, because this process is also a key asset for him, 4 weeks are realistic to control 80% of the Situation. Projects for 12 months or longer are also difficult, because then the Interim Manager is becoming more and more a part of the organization then, what reduces his effectiveness.
The costs for an Interim Manager compared with hiring a permanent manager are not higher - but his use is more flexibel. The Perm Manager in Germany for example has 30 Day a year vacation, loses 10 days a year for sic leave, gets a Company car including free Petrol, uses 5 days for training, causes Fee for a recruiting consulter and - worst case - the Same costs for a severance package. Furthermore the company has to pay fringe benefits of about €12.000 a year. Last but not least is the Interim Manager in most cases overqualified for his project, what brings additional know how and fast results to the customer.
I plan at least 1 month time after I finished a project for lessons learned and idenfication of know how gaps. That is a lot more time as a permanent manager does have for this content. I look to improve the processes to increase my performance for the next projects. At the same time I close skill gaps, that I have recognized during my project in regard of technology, customer and market developments.