How to change network interface connection timeout in systemctl

When I boot my Debian system, its startup hangs for approximately 2 minutes on the line: Loading, please wait... [ 5.191043] systemd-fsck: /dev/mmcblk0p2: clean, 234192/873120 files, 2335263/3491328 blocks [ 6.645242] intel_rapl: no valid rapl domains found in package 0 [ 6.643682] systemd-fsck: fsck.fat 3.0.27 (2014-11-12) [ 6.664844] intel_rapl: no valid rapl domains found in package 0 [ 6.660702] systemd-fsck: /dev/mmcblk0p1: 3 files, 33/130812 clusters [ **] A start job is running for LSB: Raise network interf...38s / no limit) Eventually, it gives up on raising the network interface and continues booting. **Question:** How can I change the timeout used in systemctl, so that after 15 seconds it stops trying to raise the network interface? **Additional info:** the source of the problem is the use of a wifi USB dongle on this machine. Sometimes the dongle is there, sometimes it's not. When the dongle is not plugged in, I expect the network interface to not be raised, and I just don't want to wait for too long.