Never!

I have kept in touch with her for more than 25 years XD

To me the most discrete way would be to hear her news from time to time via a common friend.

It depends greatly on circumstance. If you parted amicably, there is no reason to not remain on speaking terms indefinately. If the parting was not so civil, perhaps, you should consider forming a wall of separation for the benefit of you both.

If you enter into a new relationship, and you continue to keep in touch with an old BF or GF, then it is like saying to your new BF or GF, "I don't really love you - I really want to be with this other person". Even if you want to be with the new BF/GF, this is what they will think. It's a matter of Love and Respect for your new partner that you severe old relationships completely. That's my opinion :)