There is never a right time. Actually, the whole thought of the road ahead is no doubt filled with fears and anxiety. But the other option is to stay with a person you don't know anymore, and the silence between you says more than any diatribe of words.
But ... actually I'll make that a big BUT make sure you have exhausted every other channel to stay together as separation actually stinks. Have you seen a counsellor yourself to work on your insecurities or fears before seeing one together? Does your partner need to work through their own issues first to really be able to express themselves properly? If you have tried, but the light is definitely lost, then you're better off walking down this tunnel called separation instead of staying in darkness for the rest of your life.
And don't think waiting for 'when the kids have finished school' is a good decision at all. They are so impressionable in their youth and they look to you both to show them how to communicate and to love. So if you don't have that, how are they expected to mature into adults with the right relationship messages? Children are resilient, so as long as you put them first in this process, they will be ok.
You definitely need to act if you've been lying to yourself and your partner regarding your sexual inclination. If you are LGBT, and have hidden behind the veil of marriage or a long-term relationship with the opposite sex, your struggle will just get harder and emotionally draining.
And if there is any form of physical or mental abuse, the right time is now. There are so many support services available who can help you release the grip of your manipulative partner.
Just make sure you have thought through everything and don't be reactive as the cost to your emotional state and that of your children, plus your bank balance, can be excessive.