It is very difficult for anyone to tell you exactly how wide or tall a particular plant is going to grow. Soil, rainfall and exposure can all make a difference. This plant can be grown as a shrub or tree, and your clear central trunk suggests these may grow more upright. Nonetheless, it seems you can expect at least 10' spread. That means the trunk should be spaced a minimum of 5-7' from the property line and 10' between plants. I found this ARTICLE that has some pruning tips on keeping the shape more upright along with other info. A SF Gate ARTICLE says this variety can live for 20 years or begin to decline in half that time.

Do you want them to like like a dense hedge or like individual trees?

this is how i look into the future of a named plant: https://yippy.com/ysa/cgi-bin/query-meta?v%3aproject=yippy-com-images&v%3asources=yippy-images&query=Ceanothus%20Ray%20Hartman&;

how that all works out with your plants.. in your garden.. in your location.. is left to fate and your pruning ...

who knew there were CA lilacs.. lol ..

ken