How long does it take to upgrade the watering can?

It takes two days to upgrade the watering can. Since you can use the watering can both on the day you take it into the shop and the day you pick it up, there's normally only one day that you won't be able to water your plants. So you want to ensure you don't need to water your plants that day. There are a few easy ways to ensure this without having to resort installing a bunch of sprinklers.

• Upgrade the watering can on a day when the weather forecast for the next day is for rain
• Upgrade it on Spring 27, Fall 27 or Winter 27. You don't need to water your plants on the 28th day of these months because none of your plants will survive the change in seasons. Summer is an exception because corn will survive into Fall.
• Upgrade it on any day in the Winter (except Winter 28) if you're not growing one of the few crops, all Winter forage, that can grow then.

Finally you should also ensure that you'll be able to pick up the watering can on day the it will be available. Clint's store is closed or otherwise inaccessible on most festival days.

I should also I point out I discovered the last problem the hard way. As it turns it out not watering your crops one day isn't the end of the world. None of them died, it just delayed their growth by one day.

It takes 2 days, so you can water one day, give it to Clint before 5PM, miss one day of watering, and pick it up the third day after 9AM. So one rainy day in the middle will allow you to keep your crops growing