Games just like Clash of Clans
It depends on a lot of factors. As mentioned in the comments, let's assume Unity. The good thing is that the whole framework and toolset is pretty much done.
Take for instance the experience... Imagine one coder that never created a game in his life and an artist that never worked in games. The first week is going to look like the best idea they had. Then they'll notice they underestimated every single aspect of creating this game. They'll learn that they neglected a lot of the game design, that they forgot sound and music, that the animations don't match the speed, that the client-server messaging protocol is messing up the game's framerate.
When they finish the game, they review it, take a lot of notes and start polishing it. They then release it and notice the servers can't handle a lot of players so they need to review messaging protocols and databases. My guess is that if they didn't give up yet, which most likely they have a long time ago, they will now. Even if they don't, until they have a game with the quality and production values of Clash Royale it will take them years.
Where do I want to go with this talk? A game is all about execution. When people look at a game like Clash Royale they think "this is a very nice game and it's so simple." The game is simple and nice it's not because the game is simple and nice. The game is simple and nice because it was impeccably executed to be exactly that.
On the other side of the spectrum a development team with a strong knowledgeable dev lead and two or three developers will probably tackle a lot of the code structure and design issues in a relatively short time. I'm talking of things like classes, protocols with the communications with the server, resource management (although with Unity that is simplified), etc.
The base game is probably done in a very short time. A couple of months if that much. The rest is adding functionality and polishing continuously until the minimum viable product is ready. How long it takes depends on the amount and complexity of functionalities added and the size of the team. There's a saying in gamedev that goes like "the last 10% take 90% of the time." This is not entirely true but almost. The game itself often takes very little time to do.
