Many non-turtlers argue that it is impossible to win when one turtles. The reason being that you spend so much on stationary targets, such as turrets, that you cannot spend money on building units that can attack the other base, thereby forcing the battle into a stalemate.
Since you are a Turtler, you likely will be able to prove these non-Turtlers' arguments wrong. I hope.
Depends on the game and economic system. If resources are finite and can be recovered, or are truly infinite, they turtling can work by just outlasting your opponent(s). Let the enemy waste forces on your defenses, then ride out after the map has been drained, or your own forces are big enough. That certainly the instinctual logic that makes most people turtlers in the first place, and in some games in can work. Total Annihilation comes to mind (Seven Islands FTW).
In other games where resources are infinite but hard to reclaim, or truly finite, it's pretty damn hard. In games where the designers just don't want people turtling (Starcraft) it's not a viable strategy at all. Take all of the above with a grain of salt, as for all my love of RTSs, I lost a
lot, when I bothered with multiplayer at all.