How far in advance can you purchase tickets...Thanks.

Since nobody has answered...
I would have to check the calendar on their website, so you might as well do that yourself. If you want to stay flexible you can keep an eye on the calendar to see when slots seem to fill up.

We went there on a Sunday, and if my memory serves, we booked the tix just a few days, maybe a week, in advance.

I had looked at the website, which didn't say (or I couldn't find the info). I'll keep checking their calendar.

I looked too but didn't see anything. Give them a call.

Just for grins & giggles, I went to the Barnes Foundation and checked for tickets for tomorrow. It appears timed tix for just about any time are easily available.
https://tinyurl.com/BarnesNov30

In addition, the ticket purchasing calendar does not go beyond December 30th -- which seems to say that one can only buy tix less than a month ahead of time.

Thanks everyone!

That used to be harder to get into but I believe it has moderated since they moved the collection out of the old man's house and the residential neighborhood it was in. I have not been to the new location. It is a great collection and the "curation" made it even more interesting as it gave some insights into the owner's mind. I will have to think about going back. Do go though its a treat.