This is an interesting question, that I hadn't really considered before. I'd no idea what my answer should be, so I looked at my last two years of expenses (two years because travel and recreational equipment purchases are sporadic, and that timeframe seems to cover a good range). Over that two years, I spent about $17k (whoa! really? whew knew? Thanks again for providing the impetus to look at this...) on the non-essential categories of: clothing, crafts, dining, entertainment, garden, gifts, recreation, subscriptions, and travel. That's ~700/month on average.
Some of my personal travel is subsidized by work (for example, last year I went to CA for work, then took a vacation from there to OR, with much cheaper airfare than VA to OR would have been), so in retirement my travel expenses would be higher. Some of the other expenses in that category might be lower in the future, but I'd guess I'd still want to be closer to $1000/mo than $500/mo.