I have installed OpenERP All-In-One Windows Installation and this installation is under GNU General Public License.
As I know OpenERP 7 is an open source software. May I know whether this software is free to use or it will be expired after 15 days and need to pay for it after 15 days trial?
OpenERP is and will be free of license fee. Apart from that you need to invest either money* or your time** into getting it to work efficiently.
*Money would be needed to pay people to help you with implementation, maintenance or customizations of OpenERP
**Your time would be needed whether you want to figure out all this by yourself
this discussion offers some interesting statements about this issue I think
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