Leave to cook on low heat for about 20 minutes until it is "stewed". Check by stirring: you should no longer be able to see individual pieces of rhubarb. They should have disintegrated into a smooth purée.
Your stewed rhubarb is ready. Leave to cool, then keep refrigerated in a sealed container.
Remarks
Taste the fruit before it's completely cooked to see if it needs a little more sugar.
If you'd like a smoother result, you can blend the rhubarb briefly, but it's hardly necessary.