It all depends.
It depends on what you are going to text your ex girlfriend. It depends on how often you send her text messages. It depends on how she feels about hearing from you.
Most of the time, it's better to NOT send her a text message.
Think about the things that you want to text her. You probably want to tell her that you miss her. You probably want to tell her that she is beautiful. You might even want to ask what she is up to, so that you can get a sense of whether or not she has move on or if she might still be interested in you.
Here is the problem with that:
It's not the right time and using text messages as a way to keep tabs on her is not going to come across well at all. It might even come across as being kind of "stalkerish."
There is a time and a place for texting your ex girlfriend.
After you have already gone through a period of no contact and you are working on winning her back, it can be a good idea to send her the occasional text. Maybe you want to just say hello or you might want to make plans to spend some time with her.
What you want to keep in mind is... most of the time, it's better to hold off on texting your ex girlfriend. You can't take back a message that you send to her and there is a good chance that you probably will end up sending her a text that you wish you could take back.