The STP timers are calculated based on a default of 7 hops diameter for a switching fabric.

That means that fromany point of your switching fabric a packet will traverse not more than 7 switches in order to reach the edge of the network. This applies where you have STP running.

Not sure if RSTP required this as the need for STP timers are negated by the very design of this protocol.

If you do not have redundant connections handles by STP I don't think there is a limit but for sure the chained structure is not the best desingn if you have many switches.

HTH

Cristian