I think Genki 1 should be done in a 3-6 months span (workbook included).
That it’s basically one chapter a week or even as little as one every two weeks.
It’s no fixed number by any means, but it’s very basic grammar that will let you start consuming other materials: graded readers, native (very basic) media, etc.
Besides that you run the risk to start forgetting last lesson when doing your next one. So it’s like one step ahead… and two backwards every week.
I don’t plan to demotivate you, but there’re very interesting things to do with grammar besides textbooks.
I posted a bit about my Genki routine here. Put yourself daily goals. Dividing each chapter into smaller chunks will prove very useful.
PS. After Genki I, my whole grammar routine has changed. You can check the recommendations people provided me here as well. Maybe some of those will be useful to you too.