Is it dangerous to use a mast cell stabiliser eye drop (e.g. opticrom) for 6-12 months, three times a day?

No: I know of no problems using it for a prolonged period. If you live where you have problems with allergies year long and it helps you by all means use it.

Cromolyn is safe: Opticrom is a brand that has sodium cromolyn as an active ingredient. There are no known complications of taking much higher doses of cromolyn internally in inhalers or in capsules. Taking the Opticrom or any of the generic eye drops that have this ingredient are safe to take as long as you need it. Also, It works better if you take it before exposed to the allergen to which you are sensitive.