Recommend slide maximum

Generally, the recommendations appearing in the forums have been focused on packaging larger courses into smaller segments. For mobile use, this makes the most sense for managing file sizes and downloads.

As for your specifics, you should (at a bare minimum) assess your audience: their needs and usage patterns; your environment: the LMS, networks and user workstations; and your customers expectations.

You must also consider your content, objectives, and assessment strategy.

Your message is written in terms of slides, but you don't mention contact time or use of interactions or video. You did mention animations, but that would be passive time, little interaction, one download, one play. If they are clicking through your 65 slide presentation with minimal retention or delay, 15 minutes may be reasonable.

As an extreme example, if you are delivering new concepts, that must be contemplated, analyzed and assimilated with existing knowledge... each slide could take as long as 3 - 5 minutes. Multiplied by 65 slides, this is an unreasonable time for a single sitting. I would that that would be an issue!

If this is a single course, build it and test it. Early, often, with end users, and from their work location. If you test by having users 'stop by' your desk, and you're running off a development server or your desktop, you're not truly testing all the variables that will determine the courses usability and their user experience.

If, on the other hand, you publish a course to the actual lms, visit the user at their workplace, use their workstation, on their network... you have a reasonable chance of seeing how it will really work for them.

Except, you are only testing with one user, you won't see how it looks when a manager or director sends a message that everyone must take this course by 10 am, and suddenly you have 500 -  5000 users trying to take the course at one time.

Also I meant "recommended", sorry.