implementing autostop condition on packet capture!

Im doing a live capture using pcap librery..

pcap_t pcap_open_live(const char device, int snaplen,int promisc, int to_ms, char *errbuf)

then i dump it into a file...

pcap_dumper_t pcap_dump_open(pcap_t p, const char *fname)

void pcap_dump(u_char user, struct pcap_pkthdr h,u_char *sp)

and then dissecting it using libwireshark..

now i want to put a tiimer based autostop condition on packet capturing and dumping.. coudnt find a way how to do it.. as pcap doesnt allow.

suggest me the ways pls!