When to close out an investment when it is in a pension account I can't access for 20 more years...

I am in a fortunate position to have quite a healthy investment balance at 35 years old inside my SIPP (UK pension account). However, I definitely struggle with knowing when to realise a gain and close out a position, for two reasons.

1. I can't access that money, its in the account for at least another 20 years, so what is the point of closing a position if I believe it will go up in the next 20 years.

2. When I don't have a plan of what to do with the cash once I close out a position.

These are the sorts of things I think about, I am just not sure what the process should be. Any thoughts would be much appreciated.