but I dont know when next years time table is out.
It's available already - go via polrail.com :)
Warsaw to Szczecin by train should be about 6 hours with a change in Poznan, or about 4:30 if it's a direct train. Pricewise - depends, you can do it as cheaply as 75zl or so, or could be 200zl for the best trains.
I'd recommend the train personally - there's two IC departures (reservation mandatory) a day, one TLK departure (reservation mandatory for part of the route) and two IR departures (no reservations) - off the top of my head, IC should be about 150zl, TLK about 65-70 and IR about 50zl. I'd recomend IC trains if you're travelling with a family as they give peace of mind and are very comfortable. TLK or IR can be hit or miss.