How best to deter early knight advances without weakening position/losing time?

Do you mean something like this? If so, your best course of action is to just develop your pieces as normal unless there's a threat to deal with. Once you castle and have a sound position you can choose to drive the knight away with a pawn move, but I would not make moves like h6 to prevent this.

Are you talking about the Fried Liver? It's 1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4. Ng5

It's one of the most feared attacks on chess.com

Don't play 3. ...h6, as it's not stopping it.

4.Ng5 wins a pawn, but black has little to fear; 4...d5 5.exd5 Na5 gives black good chances for his pawn. 

Knight 'advances' in other lines simply waste time, and the other player can take advantage by constructing a lead in development, or using the tempi afforded by clumsy knights to make space-gaining moves.

I think these knight raids are often to trade for a bishop. The thing is,  does it  lose time and it may be that the loss of time is not worth it. Is the long term bishop pair worth the loss of time ? Well it depends on the position's charactestics !

If the knight is in the proximity of the king should it be booted away ? ("To be or not to be that is the question " :-)  ). You just have to analyse it to be honest. I remember in his fantastic book "Logical Chess",the chapter the Kingside Attack, Irving Chernev was catagorical in not moving pawns in front of the castled king - the offending defender who tried to kick away a knight, bishop etc was punished by a timely sacrifice. Schematic thingking is useful and I remember following Chernev to the "T", but in retrospect, realise that chess (thankfully !) can't be pigeon holed - it all comes down to the demands of the position.

Be mindful that your bounder of an opponent intent on irritating you with his knight insurgency might be brought to book with counter action elsewhere on the board ! :-)

well the knight has no useful  squares and the bishop is blocked from c4

Bad moves hurt your opponent's position on their own.  Sometimes there happens to be a tactical punishment, but many times it doesn't take any special sequence to punish them.  Just play normal moves and you'll find they simply have a worse position.  Trying to immediately punish an unorthodox move is a beginner mistake.

Although true, this advice isn't particularly useful.  As others said play over example games (master games) to get a sense for the middlegame plans for both sides and it will help the opening moves make more sense as well as help you know which moves are ignorable (the ones that aren't in keeping with the middlegame ideas).

How to deter early knight advances......?

2 parts to this. Only the good advances should be prevented. If it advances and moves to the edge of the board you may be in a position to attack it later. Or maybe with it being at the edge you can attack the other side with more force in your favor. Or maybe the knight advance removes a defender to a square.  Or possible allows to you to exchange your useless knight for your opponents.

So this is coming down to the situation on the board no matter which way you look at. Plus whenever you move a piece you weaken your position somewhere. Controlling the knight advance is the best way but you have to determine if that is what you want to focus on when you might a chance to win a piece or 2 pawns in the future.

Plus are we talking in the Opening or the End Game?