I need tips on how to better estimate development time for games.
Experience and training.
The way I most improved my estimation was being the only programmer on the team. I estimated all my programming tasks and was held accountable -- via extra hours -- for my estimates.
If you aren't accountable for your estimates in a very real way it is difficult to be motivated to improve them.
Also if I were to give you a project like this, how long do you it would take to develop it from start to finish? Assuming there is only one programmer and one artist and the game is being developed in Unity.
Features are very terse, there isn't enough detail for good estimates.
Endless runner, IOS & Android, Unity engine, single player. By itself if you know what you are doing and want to make a potentially viable commercial product, I'd say bare minimums of 4 months programming and 3-4 months art. That's for a terrible product that cannot compete commercially but at least is not embarrassingly bad on the marketplace. If you aren't already experienced and don't know what you're doing I'd say at least 8 months programming (or far more, depending on the programmer's skills) and 4-8 months artwork, since the artist may have to rework things many times.
Online cross platform multiplayer for 4 players is going to add about two months programming if you really know what you are doing and your app is built for it up front, easily six months or more if you don't or the app is wrongly designed for it, and you'll need to include QA because bugs are hard. Artwork is UI, so far less time, perhaps a few weeks.
The random powerups are not that difficult, maybe another 1-2 weeks programming and whatever time is needed for art depending on visual complexity, perhaps a few more weeks if you have many diverse powerups or they do different things.
The link to facebook could be fairly simple if it is just posting directly, there are components you can buy that do almost all the work, add two weeks if you're using one of those, one if you've used them before and know what you're doing. Otherwise add in about a month, or two if you are learning how to post everything for the first time. If you want more than that, maybe full facebook integration and all that jazz you can add a full development year or even multiple development years.
Same for twitter. If you're just posting, and using a plugin, or just posting using their direct API, about the times mentioned above.
In-game chat basically comes for free with the other networking. Maybe 1-2 weeks for UI if you know what you are doing, more time if you have to learn how to handle UI and text input.
Friend invites, friends list, and join-in-progress all mean trouble unless you've got a lot more infrastructure than you are hinting at. Far beyond the scope you're mentioning.
Global leaderboards are a huge risk, I wouldn't touch them. The simple solutions are the most hackable, and they tend to become cesspools.
Having in-app purchases itself isn't too bad but you didn't list what it is that is going to be purchased. If you use a good cross platform library you won't need to write anything yourself, just hook in to a cross-platform entitlements system.
Ads will depend on the types of ads you use and the libraries they provide. Anywhere from one month to six months to implement, depending on your skill and experience.
The bare-bones "I can show my friends I made a game" endless runner could be done in about a month of programming work, about the same in art. The "I made a product that has the tiniest chance of being commercially viable" is anywhere between one to three years of programming work and anther year or so in artwork.
If that wasn't enough, the market for endless runners is not favorable to newcomers. The niche is saturated and even an amazing product is unlikely to succeed.
