I grow garlic and onions for food, not for companion planting. However, garlic is reputed to repel aphids and perhaps other pests so if you have more than you need there is certainly nothing wrong with trying it out.

Companion planting is more of a mythology than science so you are unlikely to find any good information on how much to plant in order to realize a particular benefit.

Also, the idea that large cloves produce the largest garlic bulbs is not well established even though it is conventional wisdom.

If your goal is to assist roses more than it is to get great garlic then by all means leave it in the ground all year. It will grow year after year after year.

Why not just grow ornamental garlic near your Roses?

I don't really agree with the statement about Companion Planting being more mythology - as I interplant Queen Anne's Lace around my garden where I don't want aphids - and Ladybugs are absolutely certain to come. It looks good near roses, too, IMO, and keeps blooming a long time with dead-heading. You can pull it up at the end of summer...and chop the roots for stews.

I have been planting garlic (for food) for the last 2 years. Both have been doing well beside each other. I never need to seed my garlic. It some what seeds itself. I forgot to seed my garlic after harvest last summer. By late fall, I had lots sprouting. And it looks like this year, I will have the biggest bumper crop!

I have planted garlic chives near some of my roses and while I can't honestly say it has helped deter aphids for sure, it really looks great when it sends up those lavender blossoms