My youngest who plays ECNL is a U-13er. The travel at this age is only regional, that means we travel the Ohio Valley Region. As she ages, ECNL requires teams to play in regional tournaments and eventually the national tournaments. Much of this depends on how good they are also, but the requirement for regional tournaments is there regardless of age.
At her age the commitment is spring and fall. This changes when they are at the high school level, ECNL does allow girl's to play HS so at that age there isn't ECNL in the fall, but not sure how that works if HS girls play in the spring.
Our regional season ends second week of November, and doesn't start again until March. Between November and March is winter training, friendlies, and tournaments if there are any.
We are still filling in the pieces as we learn more about ECNL, because our club is one of the newest members. We are part of the 13 added. If you check on rosters at the D1 level, many of the girls will have ECNL or CASL next to their club name. If it doesn't say ECNL, there is still a good chance that team is part of ECNL, just not stated in the player bio.
Because women's professional soccer in the US is not very financially stable, they cannot create pro team development teams. But MLS may be working to create a women's version. Not sure on that.
There are many girl's selecting to National team camps who play ECNL.
http://www.eliteclubsnationalleague.com