There are certain times in the early game when you should be on "high alert" for a jungler to come into your Lane. For example early level 2 ganks, after they completed their first jungler route, etc.
Is it possible to say "at 3 minutes top lane should be extra careful as that's when the jungler reaches there" or is there just too much variation in jungler clear times and routes for that to be useful?
You pretty much got the biggest one but...
In general, you want to watch your minimap. If you haven't seen the enemy jungler in awhile, assume he's about to gank you. If you are toplane and you see the jungler bottom, take another look at the map in about a minute. You aren't likely to get ganked before then. If the enemy jungler ganks mid from topside, and leaves assume that he will be up to you in the next 15-20 seconds until you see another appearance.
Using those general rule of thumbs, play aggressive when you know you have a good window without the enemy jungler. If you don't know where they are or they were mid on your side assume that they are going to be ganking you soon. On my 2nd or 3rd back I'll generally grab an extra ward beyond my trinket. I'll usually wait to ward until I see the enemy jungler since he could be in those bushes waiting for me to facecheck as I ward. Once I see him, if he's far bottom, I'll wait 30 seconds to ward (this is assuming I'm top). If he's mid, I'll ward immediately and like I said, if I don't have vision I'll place a ward if I know I'm pretty safe (my laner is pretty far away) otherwise I'll play passive until I'm given an opportunity.
An extra tip, is that people will often place a pink in one of the solo river bushes near mid or behind their red buff to get some easy vision. (You are more likely to lose the pink in the river, but it's a lot more valuable for vision. Essentially it's a bit of a risk factor if they find it or not.)
There is no formular when to watch out since game-balance changes to fast. It is not like in starcraft2 where you can actually time enemy strategies. That being said, here is some general advice:
Most junglers won't gank you prior Level 3. And they (mostly) level slower than champs on lane. So you could start to be prepared once you hit level 3.
Get wards ... the safest gank prevention method :)
Talk to you team and call the jungler when he is seen.
Play jungle yourself. A jungler knows how and when a jungler will
gank you.
I hope this helps.
Best Regards,
zTonic
Lets say you are top lane and the enemy is on the blue side of the map. If the enemy jungler starts at his red he should clear his blue around ~3;30 and hit level 3. meaning you can expect a gank top to hit around 3;50.
after that its all about warding and map awereness.
I guess you can add some more points here.
First of let's start with early game. You should pay attention to the enemy top and bot laner. Are they low on mana? Did they come to lane later? If you pay attention to this you can decide whether their jungler started at red or blue. According to this you can plan your defense. An important point here is that the jungler out levels you in the first 3-4 minutes of the game. He is level 3 and you are level 2 if he ganks after both buffs. Play defensively or get vision!
Always hug the side where you do not expect the jungler to be. If he started blue and takes red to get level 3, hug the blue side of your lane and the other way around. Timing enemy buffs or keeping them in mind is another important point. If you see that the enemy midlaner gets blue you should be careful in bot / top lane respectively because you know the jungler is around.
The best wards are deep wards in the enemy jungle, in my opinion, because you get warned earlier and can back off accordingly. Deep wards at his red or his blue or at the jungle entrances from mid lane. This will give you time enough not to die to ganks.
