It's ridiculously hard to come up with an answer to this seemingly simple question. I've tracked my time usage pretty meticulously by version number, but for most features I had to develop the server-side, client-side, website and network simulator code simultaneously. Developing this (or any) game I think isn't a process where you build the server first, then build the simulator and then build the client. It's building v0.0 for all parts, then v0.1 for all parts and onwards where all parts gain features in lockstep.

For Angeldust I've done dozens of updates, each and every single one building upon the existing base. Looking back it's amazing to see that the product evolved in an almost linear fashion from v0.0 all the way to the current version without ever having to drop or redo features. But because the product evolved as a whole I can not really distill a time estimate for only the "client side engine".

I mean: would you consider testing the client side engine part of client side engine development? For testing you need a working server. For which you "need" a working network simulator. It's dependencies all the way down.

And how about translation work for in-engine strings, updating shared libraries and frameworks, issue tracking, writing the asset conversion toolchain (which needs assets, which need integration). It's very hard to come up with any solid number. Maybe if you can severely limit your question's scope I can come up with an educated guess.