You know, it depends on a situation. For example, my husband Ron sent me flowers for the first time in one week (!) since we met online: it is because my birthday just happened to be that time :)
Another friend of mine got her first flowers from her western boyfriend in three weeks after they met online. Again, it was sent to a special occasion: an International Women's Day which is a big women's celebration in Ukraine.
Another one western guy sent flowers to another my friend after two months since beginning of their online correspondence. There was no special occasion, it just how he felt like :)
So, it is absolutely individual choice for every man, when to make a nice surprise to his distant beauty.
What else western men need to know: you better not to send an even numbers of flowers (2, 4, 6, 8 etc) to your Ukrainian lady, because in Ukraine even number of flowers means a funeral. An odd numbers of flowers (1, 3, 5, 7, 9 etc) mean a life, so it is absolutely appropriate for a gift.
However, if a Ukrainian woman is going to be married to a western guy, she has to learn western standards too and she has to use getting a traditional dozen of roses or dozen of other flowers over here, on the West. I myself am a Ukrainian woman, but since I got married to a Canadian man and moved to Canada, I feel absolutely all right getting dozens of flowers for my birthday or other special occasions.
On the picture above: my Ukrainian friend Lilya just got wonderful roses and bunch of balloons from her distant admirer.