Agree with tersky. The only thing I’d add is that some genetics are lankier than others (usually sativa) so the height isn’t really a factor in when to top it is just the number of nodes. If all of your plants regardless of genetics have really spaced out nodes (leggy), as tersky said, your light is too far away or not strong enough.
Edit : if you just meant how big/well defined should the sixth node be before you snip, if the plant is healthy I do it when that node is still quite small. Sometimes I do it when just the fifth node is visible. Let the plant decide based on how well it is growing.