I've created a necromancer, and was wondering at what level I should go from the Mage stone to the ritual stone? ..also, should I use archery with poison arrows (read somewhere this was a good build?)

I would keep the Mage stone until you're at least level 25. Then switch to the Ritual Stone.

And I don't think a Mage (especially a Necromancer) would use a bow. That's why you have your summons and a follower which is a big help.

or, you could always use the Aetherial Crown and have access to the powers of both

I don't know what level you should switch to Ritual Stone. I personally did it as soon as I found the stone.

As far as archery goes - while I don't use Archery in Skyrim, I have used it in Morrowind and Oblivion, and Conjuration with Archery is a very good combination.

I don't believe in the idea that says "a mage wouldn't... "

That's the beauty of Elder Scrolls games, you don't have to limit yourself to what other RPG archetypes limit you to. You can do whatever you want to do and it's all valid.

I personally thing the necro archer is a really fun build.

see, to me most weapons would not make sense for a Necromancer simply because of the potential damage to the body.

since Necromancers experiment on the corpses they acquire, and use them as soldiers and guardians, to me Frost Magic would make the most sense. it would not leave any visible lacerations, would not char the corpse like fire or lightning.... worst it would do is give a little frost bite or freezer burn....

I suppose if you're roleplaying that deep into it, that's one way of looking at it. I personally don't, so slicing and dicing enemies with dual wield swords, or turning them into a pin cushion of arrows, or charring them to a crisp with flame magic, doesn't bother me. But like I said, if you're roleplaying that deep, then I can understand why someone wouldn't want to use weapons.

But then again, that comes back down to designing your character your way, which is why Elder Scrolls is great

I have built a Bound Sword Dual Wielder using just the Warrior Stone, and another with the Mage. Both characters level Conjuration fairly evenly despite having different Birth Stones, They both had Mastery of Conjuration in the mid twenties. I had a character that used just a Bound Axe achieve Mastery of Conjuration by Level 27, but I was also using Atronachs regularly.

If You are using Destruction and Restoration then stick with the Mage Stone to offset the slower levelling Skills, the Ritual Stone IMO is more useful at lower levels to gain access to free Meat Shields during mistakes.

Archery is a good poison delivery system as long as You do not hit Your Minion(s).

I usually keep the mage stone on my mages until I feel like I start to get ahead as far as power goes. Thats really the whole point of the mage stone, level up your mage skills faster. I agree with Shadow Titan 17, level 25 is a good cutoff.