You don't need a shih tzu expert at all -- any of us can answer this.
They do not have periods - they have heat cycles (estrus) which is exactly the opposite to a human period. It is while she is bleeding that she can get pregnant so watch her like a hawk and do not allow her outside unsupervised.
This will last 3 weeks - don't think it is over when it goes clear and straw coloured at about week 2 as this is her most fertile time.
When she can have puppies depends on your dog and when you have done EVERYTHING necessary to produce strong healthy good breed standard puppies. Sorry to say this but judging by your question I'm afraid that is NEVER.
Please read all the sites below which you have until she is 2 yrs old which is the earliest she should be bred. Hopefully by then you will realise that you shouldn't be breeding at all.
http://www.k9web.com/dog-faqs/breeding.h…
http://www.woodhavenlabs.com/justonelitt…
http://www.woodhavenlabs.com/breeding/br…
http://www.21stcenturycares.org/backyard…