The poinsettia plant will stay alive for around 6 weeks to 6 months although 6 weeks is about the most they stay alive for even when properly cared for.
Also poinsettias require complete darkness every night for around 10 weeks before the bloom will be triggered so be sure to let the poinsettias get some darkness and if they're indoors you can place them in a dark room or closet where it's dark at night to trigger the blooms.
If you're lucky you may be able to get the poinsettias to stay alive all year round with proper care and sunlight when needed but usually they only last for around 6 months time at most.