At what point will Sims 3 start lagging?

@SharketteLaw92  Lag has a number of different causes, especially when you include the varying definitions of lag commonly used.  Some have nothing to do with a player's third-party content, e.g. weak hardware, a system issue not caused by hardware, or a poorly made  or overcrowded world.  Some of this can be addressed in various ways, although upgrading or replacing a weak computer may not be in the picture.  Sometimes the save itself is overloaded—the game engine can unfortunately only handle so much before performance starts to suffer.

Some lag is caused by mods or cc, but it shouldn't happen at all with well-made content.  When it does, it often seems to be random, but there's usually an identifiable cause.  For example, a bad cc item may cause no problems at all until a sim is wearing it, or the "wrong" sim is wearing it, like when an item is miscategorized as being valid for all age groups.  The solution there is simple: remove the bad cc, and the problem should go away.  That makes testing easy too, since you should see results right away.

Mods are more difficult because by the time you see lag, they may have already corrupted your save past the point of repair.  That's certainly not always the case, but it's a distinct possibility and even an inevitability with a couple of popular mods.  If you want to make sure your particular mods are safe to use, the best place to ask is on the NRaas site.  You could list your mods and ask if anything should be removed; the people there will recognize any potential issues on sight.

For the rest, when the lag happens, a little investigation is in order.  Sometimes all you need to do is quit without saving, and everything's fine again; sometimes it takes a bit more intervention.  But on a strong-enough computer with no bad mods or cc, you should always be able to address the issues that appear without having to deal with permanent lag.