You can dig it -- and much more -- out of the drive_resource list-column.
This should get you started:
library(tidyverse)
x <- googledrive::drive_find(n_max = 3)
x %>%
mutate(modified = map_chr(drive_resource, "modifiedTime"))
#> # A tibble: 3 x 4
#> name id drive_resource modified
#> * <chr> <chr> <list> <chr>
#> 1 googlesheets4-design-exploration 1xTUxWGcFLtD… <list [32]> 2018-03-1…
#> 2 Internship Application 1ZioYgbU1bzS… <list [30]> 2018-03-1…
#> 3 2018 Webinars 1WIf9VqQdGsB… <list [32]> 2018-03-1…
Created on 2018-03-15 by the reprex package (v0.2.0).