I just recently got my last guardian to Mythic +4, so now at level 6 my guild chests are equal to gem keys. In light of that, I'm thinking I'll stop using them. When a new mythic drops each month I'll burn my real gem keys, then up to 100 VIP keys to get one, but only use guild chests if it doesn't drop. That way in 3-6 month I'll have plenty of guild keys to mythic out any remaining GW troops.
If the guild finishes 9th that is 32 per war week. 3 events in 4.5 months would give me 96 copies, enough to ascend to legendary. Only guilds that finish in 1st at least once and 2nd or 3rd the other two times can reach 191 for Mythic.
If it is 4 cycles, which equates to 6 months per set, you'd need to average 3rd place to reach 191, but with more flexibility. Like finishing 6th three times and 2nd once would still work.
In a nutshell, if you aren't in the top half of the top bracket week in and week out, at 6 month (4 cycles), don't expect to see mythic. Anybody in 2nd bracket will reach legendary. Anybody in 3rd to 20th bracket will reach epic.