If the list grow too long, I prune it. Then one of two actions happens. First, if the item has some value I place it in my reference files ... these files get reviewed yearly. So effectively I've given myself another 12 months.
If there is no value (now or in the perceived future), then it's trash. And trash goes into the bin.
Another thought, is to by pass the someday/maybe list and place these items directly into the reference system. That way your list should be manageable and only need light purring once in a while.