create a systemd startup script that delays 30 minutes

I tried to create a systemd startup script that starts an rkhunter scan 30 minutes after system start of my laptop, like this:

[Unit]
Description=starts rkhunter and displays any findings with zenity

[Service]
ExecStartPre=/bin/sleep 1800
ExecStart=/usr/local/sbin/rkhunter-check

[Install]
WantedBy=default.target

But this fails with a timeout error Job for rkhunter.service failed because a timeout was exceeded. It seems like the ExecStartPre does not work like this. How do I solve this? A perfect solution would be: - first run 30 minutes after boot - repeat every 48 hours (in case you never shutdown your laptop)