At what age can you leave two kittens alone during work?

I got one kitten back in October. It took me awhile before I left him out at night and during a full work day, about two months.

I got another kitten friend for him at the end of December. I’ve been able to leave her out a little bit quicker than I left him, since they have each other.

He is 7 months, she is almost 6 months. I just don’t want to rush it; right now I’ve been coming home every day during lunch.