Yes you can take antihistamines forever.
Most people can safely take take antihistamines forever or for the rest of their lives.
However not everyone needs to take antihistamines forever and some people may only need to take antihistamines for a short while until the allergies and symptoms go away.
Before taking take antihistamines forever you should consult your doctor but if you do need to take the take antihistamines forever then it's usually safe to do so.
I personally only take take antihistamines whenever my allergies are flaring up and then I stop taking the take antihistamines when the allergies have gone away.
I hate taking any medicine long term as they can cause more problems.