If you are taking it orally as a tablet, the maximum daily recommended dose as labeled is 80mg. So that's 63% above the recommended max, for a fairly dirty drug. I would be much more concerned about the dose right now than how long you've been taking it, although long-term exposure at such a high dose is certainly a potential safety issue.
. How did you get up to this dose? Depending on the answer, I would consider, _only_under medical supervision, tapering down (the recommended process is dose reductions of 10mg every 2-3 days and you would never want to go down any faster than that but you also need to be monitored since the process can be fatal).
Are you sure that's the dose you're on? Has your spasticity been that severe? Since spasticity tends to wane as nerves die, anyone on baclofen or tizanidine should have their dosage re-evaluated periodically anyway. Some of the symptoms you've ascribed to ALS or other conditions may actually be byproducts of these drugs.