When would one answer “amen” to his own berachah?

Generally, we do not answer “Amen” to our own berachah, but the minhag is to say “Amen” at the end of a berachah that concludes a series of berachot. For example, the tefilah of Yishtabach, ends by reciting, “Melech Kel Chai Ha’olamim Amen.” Yishtabach concludes the section that began with the berachah of Baruch She’amar, and we therefore end with “Amen” as this concludes a series of berachot.

If however, the series of berachot would be similar, as we find in the berachot recited at an aliyat ha’torah. Then we would not answer “amen” to our own berachah.