To get the total login hours for a given user define the function:

login_hours() (
    total=$(for f in $(ls -rt /var/log/wtmp*); { ac -p "${1:-$USER}" -f $f | tail -n1; } | awk '{sum+=$NF}END{print sum}')
    first_day="$(ac -dyf $(ls -tr /var/log/wtmp* | head -n1) | head -n1 | awk '{print $1"/"$2"/"$3}')"
    printf "Total login hours since $first_day:\n$total\n"
)

The count starts from the first recorded day in /var/log/wtmp*.

Usage:

login_hours sergioro
Total login hours since Aug/15/2018:
9590.95