A general rule of thumb on how fast to do a given rep of any exercise, 2-3 seconds per rep will yield good results. 

If there is no specific exercise tempo listed for an exercise within a program, make each rep take 2-3 seconds. 
Sometimes within a program, there will be specific tempos to follow which will change the difficulty of the exercise. 

Tempo during exercise is very important.

Concentric / Isometric / Eccentric

C/I/E = x/x/x

Concentric – The motion of an active muscle while it is shortening.

For example – you do a standing bicep curl, and your hands are traveling upward from your hip up to your shoulder.
1/x/x = C/x/x - Concentric

Isometric – The joint angle and muscle length do not change during contraction. Zero range of motion.
Example – Holding a Plank position or holding your hand at the top of a bicep curl near your shoulder.
Isometric is very easy to understand. It’s the point where you transfer from Concentric into Eccentric or Eccentric into Concentric. 
x/1/x = x/I/x - Isometric

 

Eccentric – The motion of an active muscle while it is lengthening. 

For example – you do a standing bicep curl, starting from at the shoulder your hands are traveling down towards your hip.
x/x/1 = x/x/E - Eccentric

You will normally see Tempo written like this - Bicep Curl 4×12 - 2/1/2 - 3-minute rest

A tempo of 2/1/2 would be 
2 seconds - Concentric
1 second - Isometric 
2 seconds - Essentric 

Squat – although our first number is concentric in tempo programming the squat first starts with eccentric movement.
Keep in mind, as opposed to the bicep curl where it first started with concentric movement and finishes with eccentric movement.

Squat - 3×8 - 1/2/3

A squat would be performed as – 3/2/1

The squat starts at the last number being… Eccentric, Isometric, Concentric. This is because the squat first starts with eccentric movement where the muscle lengthens.

Although program writing will always be in Concentric, Isometric, and Eccentric, order. Depending on the exercise you may need to reverse the order or keep it the same.  

Bicep Curl – 1×8 – Tempo 1/2/3

Perform as – 1/2/3

vs

Squat – 1×8 – Tempo 1/2/3

A squat would be performed as – 3/2/1