It’s recommended to change the springs when:

1 There is a wear or break of the suspension rubber pads.

2 There is a  premature wear of shock absorbers, ball joints and tie-rod ends.

3 The vehicle tends to open more and more when taking a curve.

4 There is an intensive wear of bushings (oscillating arms or control arms).

5 The vehicle is inclined or low.

6 There are visible cracks, broken turns or support marks (the two first turns and the two last ones are normally nearer between each other than the central ones)

7 There are several oxide spots (to remove the oxide does not eliminate the problem)

8 There are visible wear marks on the turns of the springs.

9 Whenever a fitting to the vehicle is added and which modifies its weight (G.N.C, Music Equipment, etc.)

10 Though, you do not realise any change in your vehicle, replace the springs every 80,000 km as a maximun, since a weary spring may have an identical appearance of a new spring without having the same dynamic reaction capacity.

THE ROLE OF THE SPRINGS

The specific role of the springs is to support the vehicle weight and its load. They keep the car levelled and prevent the passengers from receiving vibrations suffered by the wheels, whether the vehicle is stopped or running.
When a wheel overcomes an obstacle, the spring is compressed accumulating energy which will make the spring distend and oscillate several times until finding its balance. The shock absorbers are used just to avoid an excessive number of oscillations.
Increasing the resistance to the sharp movements of the spring, the shock absorbers limit the oscillations, restoring quickly the vehicle balance.

A good set of springs must have two essential characteristics from the physical and operational point of view: RESISTANCE and ELASTICITY. RESISTANCE is easily verifiable, since the spring is broken when its resistance is surpassed.
On the other hand, ELASTICITY is not so easy to be noticed, since a weary spring may have an identical appearance of a new spring without having, nevertheless, the same dynamic capacity reaction.