How to limit the running time of a program

In addition to Gilles answer there is cpulimit tool that does exactly what you want - including modifing in runtime. Additionally it can limit to only certain CPUs/Cores IIRC.

nproc was the problem:

[root@localhost ~]# ps -eLf | grep pascal | wc -l
4068
[root@localhost ~]# cat /etc/security/limits.d/20-nproc.conf
# Default limit for number of user's processes to prevent
# accidental fork bombs.
# See rhbz #432903 for reasoning.

*          soft    nproc     4096
root       soft    nproc     unlimited
[root@localhost ~]#

man limits.conf states:

   Also, please note that all limit settings are set per login. They are
   not global, nor are they permanent; existing only for the duration of
   the session. One exception is the maxlogin option, this one is system
   wide. But there is a race, concurrent logins at the same time will not
   always be detected as such but only counted as one.

It appears to me that nproc is only enforced per login but counts globally. So a login with nproc 8192 and 5000 threads would have no problems, but a simultaneous login of the same UID with nproc 4096 and 50 threads would not be able to create more because the global count (5050) is above its nproc setting.

[root@localhost ~]# ps -eLf | grep pascal | grep google/chrome | wc -l
3792