https://www.mail-archive.com/bitcoin-dev@lists.linuxfoundation.org/msg09604.html

• 3months activation period, 90% hashrate threshold
• if activated, schnorr enablement is 6mo after start
• if not activated, try something else

the longer enablement delay after activation is interesting and reduces the risk from unupgraded nodes on services, miners, wallets, users and gives them time (a risk otherwise increased by short activation window)

while the short activation period avoids delays or perceived indecision resulting in a user-led UASF as SPDY8 is "fast enough", and if the unexpected were to happen and miners unreasonably delayed, then it should be easier to find consensus on a flag height activation from there. (actually not clear if BIP 8 or BIP 9, I think BIP 8 is considered an improvement due to use of block heights vs dates which created some subtle problem in the past).