The only reason why someone would time is if they want more or less ranged units. Because overall, you still get the same amount of units regardless (one for each city you have, and one for each you settle) - all it encourages is for you to have more cities.
I don't like policies with one-time effects
in general and using this policy to suddenly pop a defensive army seems cheap to me. I do like the idea of getting a free ranged unit with new cities, though - without the instant army. It still encourages settling and gives makes your expansion more defensible.
I'd probably go for something like that (though I don't like the name "conquest", but that's a quibble for later):
Opener: +33% vs. Barbarians, encampment notifications and culture for barbarian units killed and
for each conquered city (I like the Honor opener in general, but I think adding city conquest means it has some lasting effects after barbarians decline - yet it's rare enough not to do too much).
Feudalism: Garrisons in occupied cities reduce unhappiness by 25%, garrisoned puppeted cities gain +25% gold (this offsets the regular yield penalty to puppeted cities and makes it more useful for the empire as a whole, by making it available earlier, it helps you with earlier wars which cost you enough gold... might need something extra to make it worthwhile in peacetime, perhaps +1 hammer or +1 happiness from garrisons - I like the garrison theme, though).
Collectivism [requires Feudalism]: Free settler and 50% settler training speed. (free settler is too good to be handed out right after the opener, this is still a good policy)
Discipline: +15% unit combat strength w/ friendly adjacent unit and workers improve tiles 25% faster. (basically, it makes all units more efficient, including workers, as initial buy in, that should be fine - don't want to lose the combat bonus either)
Military Caste [requires Discipline]: Free ranged unit with new cities and upgrade costs reduced by 33% (gives that incentive for new cities and means it's easier to keep the garrison up-to-date).
Exaltation [requires Discipline]: Starts a Golden Age. Free great general and they are earned 50% faster and barracks give +1 culture (a free Heroic Epic is very timing dependent, this late in the tree, it's rather likely to
already have it, this might be better - this policy is a bit weaker, so I slapped on the +1 culture which kind of fits, but... better ideas are good)
Finisher: Cities increase the culture cost of policies by 33% less than normal and all cities gain free barracks (no maintenance). GG purchase with faith at Industrial. (I like the culture reduction, but I think we can just copy the 33% from Representation. Free barracks are good, it's nice for settled/conquered cities to have them immediately and having them maintenance free for existing cities is good too and ensures that even the AI makes the best use of them)
EDIT: This is mainly my take on Wodhann's proposal, I think there is a need for more things in it to help with a peaceful wide play-style that benefits military, too. Mainly: happiness bonuses, city-connection bonuses and a bit of culture.