Actually, the best way to say this is 'In twenty years.'
If you must use 'time' then this becomes the main noun, and 'twenty years' becomes the adjectival phrase describing time. Because this is not a possessive, you need no apostrophe.
You would say 'In twenty years time.'
'This will be a splendid tree in time.'
'This will be a splendid tree in twenty years.'
'This will be a splendid tree in twenty years time.'
The final sentence adds nothing to the second sentence because 'twenty years' cannot be anything but time. It's like talking about a 'human man' or 'wet water'.