Double checking the rules, magi can only retrain their Arcana and bonus feats as part of retraining class features, meaning that they cannot retrain spellcasting or spell slots.
For retraining archetypes the rule works as the following:
To gain an archetype that replaces standard class abilities you already have, you must spend 5 days for every alternate class feature you would add, subtract, or replace by taking that archetype.
Which means you will spend 5 days per ability that the new archetype replaces. It will take you some time.
A raw reading suggests that retraining into Kensai would take 60 days, as the archetype adds/subtracts/changes 12 class features. But it would make a lot more sense for it to take 25 days, as you would only have learned 5 archetype abilities by level 5 of Kensai.
Not to mention that if you want to remove your old archetype, first you have to retrain out of Bladebound, which modifies one ability and replaces another, for a an additional of 10 days. But that's not required, as both archetypes are compatible.