How do I make the time between dispensers longer and random?

Solution 1:

Randomizer

There are two parts to this question. The first part is the randomization. The simplest way to randomize redstone is to use a dropper and a hopper pointing into each other. Use a comparator pointing out of the hopper and place exactly 2 or 3 pieces of redstone dust after it. Place a single non-stackable item (such as a wooden sword) into the dropper, and add any number of different stackable items. The more stackable items you place, the less likely it is that the output (red wool) gets powered whenever you power the input (green wool).

The way this works is that the non-stackable item will take up 1/5 of the total space of the hopper, therefore the redstone signal strength will be 3 (1/5 of the max), which is enough to power the output. For a stackable item, the signal strength is only 1.

Clock

The second art is a good clock. For this issue, I suggest using an Etho Hopper clock, which you can see at the top of the picture below.

It consists of two hoppers pointing into each other. Both have a comparator pointing into a solid block with redstone dust on top. Place sticky pistons on top of the comparator and add a redstone block in between. You can use virtually anything for your output, I used torches on the side.

Place items into one of the hoppers. The amount of items determines the clock timing (0.8 seconds per item), which can be as slow as once every 4.2 minutes.

Mockup

Here's a mockup of what it could look like for your system. The dispensers are below the granite blocks.