When should I be building an army with the British? How many minutes in?
It depends on the type of game. Is it a no rules game? Team game? No rush 15, 20, 30 game?
If it is 1v1 most people rush so you should be aging and building an army as quick as possible. Most less experienced players get caught with their pants down manor booming and get attacked by a large army they can never keep up to. If you are going to try to boom for a while before you build you should be scouting your opponent to see if he is building his.
If it is a team game you will want to go with the team’s strategy whether it is to rush or boom or fast fortress.
@AromaticBag858 , for the record, there’s no No Rush 15. Just 10, 20, 30 or 40. 
Answering the question: usually there are two ways to approach the military side of your colony. First – trying to protect your booming economy early on with a small number of inexpensive troops, which correlates with your overall defensive take. This means that upon aging to the Colonial Age you focus on laying down most likely as many manor houses as possible while simultaneously having approximately 10–15 guarding infantrymen: pikemen, musketeers or longbowmen. It allows you to have a steady resource income in the mid-game to then assault your opponent. The best circumstances in which it can be applied are, naturally, match-ups against less-aggressive civilisations that can’t punish your „greed" early, like the Dutch or the Portuguese.
Second – massing up your army at the beginning of the second age to flood your opponent with your quality soldiers, when at the same time keeping up your facilities going up steadily. It translates to intensified military production from a barracks and a stable right after advancing to Colonial along with sending wood or settler shipments from your home city to enhance your resource-gathering tempo. Recommendable against nations with relatively weak period of transitioning to the Fortress Age, like the Spanish, the Chinese or the Sioux.