Hello SCOMAN.
I must admit that nearly all the broken/damaged trees that I have ever seen still have their props. There is one particular exception in Kew that has benefitted from being partially blown over. This tree also started a train of thought on ground compaction and now looks magnificent, but I digress!
I have quite a lot of Rowan in the woods that I look after, they certainly grow tall there but not particularly large diameter wise. They blow over or lean after winds. Unless they are dangerous to the walking public I quite often leave them and they usually end up on the forest floor.
In your instance you will be wanting the root sysyem to re establish itself enough to support the stem again. This may never happen enough to support the tree and if you remove the support the likelyhood is that the first wind through will blow it over again. You never know though, optimism is a fine thing! I would certainly give it five years. You need to ensure the support does not chafe the bark when it moves.
Alterntively maybe this is the time to plant a youngster. I know it's good luck to have a Rowan by your entrance. Perhaps it's time to plant something else in its place (crop rotation) and a new one in the good luck position. I would suggest another Sorbus but this time perhaps a Torminalis.
I know most of this is only speculation on my part but I hope you resolve this problem. There were some fantastic trees that were lost to the 1987 gale. Time and nature is a great healer. 24 years later there are some fantastic young trees that were planted to replace them!
Swyn.