What I would do is see how long, on average (not your best or worst speed) it takes you to write 1,000 words. Then see how much you can accomplish on the picture book in that same amount of time (again going with an average), and then multiply that by 50. So if one whole illustrated page takes an hour on average, then do 50 pages. Or you can use time – if 1,000 words takes you 1 hour, then aim for 50 hours of work.
A lot of people like time-based options (it’s very popular with revision) – I don’t like it as much because it’s harder to get excited about doing more in less time, since the time counts more than the actual progress. But it is an option for pretty much any project.