As my first time submitting a book proposal, it is required to indicate the amount of time expected to deliver the first draft of the book. I am not sure what to say because I have no idea about what the editors would consider enough time to write a book of 60k words. As I would really appreciate your insights, what do you think would be a reasonable amount of time to write 60k words book?
As @JonCuster comments, you don't provide enough context for a reasonable answer. Is this just something you want to write but haven't started? Do you have fragments or drafts already down on (metaphorical) paper?
My experience writing mathematics textbooks is that each one takes me several years, starting with rough notes prepared for a course. I never wrote a formal book proposal since each book was essentially done before I had a publisher.
I wrote a draft of a 60K book with no technical content while on sabbatical one year. I never finished it (and probably never will).
Guessing that it comes to 150 or so pages, but hard to be exact. I was once told (by someone in the "know") that a good writing plan is to "make one page of progress per day". That doesn't mean write one page and then quit for the day, but that your draft is one page longer at the end of each day. It also doesn't mean that the days will be short or easy. Some will be very long and hard with revisions.
But if you can manage that, then about five months.
I'm pretty sure that the intent was not scientific writing but that the creative bit was also done along with the writing, as opposed to just having a complete outline. But I don't think that my experience writing programing textbooks is too far different.