As you stated, every case is different. So with that in mind:
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What type of integration is required? Just CRM systems? Do you have other legacy systems/databases/reporting that would need integration?
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Are you migrating from a previous marketing automation platform?
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What is your IT department like? Do they have experience with backend integrations?
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If you don't have an IT department, are you using a consultant? If so, he should give you a scope of hours.
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If you/your IT department have no experience with this endeavor, you can get Marketo up and "functioning" pretty quickly, but the deeper integration pieces (Munchkin tracking code, salesforce insight, etc) will take time.
On one integration, we had a product marketing team that was busy with other work and couldn't dedicate fulltime to the launch of Marketo. We sat down and had a meeting with them and figured out what the most important pieces were for them to take care of. We also set up a schedule and used PM tools to make sure the integration was kept on track.
Depending on the number of pieces you need integrated and the experience level of the team, it can range anywhere from 4-5 days up to 3-4 months to be fully up and operating.