Looking at Felix tonight I can see how much he has grown. He is almost too big for the chair.

In February last year, at 4 months old, he was a tidier fit.

But by April last year, at 6 months old, he seemed to have had a growth spurt.

I just did a bit of quick research and learned from several sources that (with the exception of some breeds) cats stop growing at about 12 to 18 months of age. So the chair might do for a while yet.