Is there any estimated time frame for the validity of
JViewLegacy? When will the switch to native 3.x ones occur?
So technically you start transitioning to the new MVC classes now - there's nothing stopping you. For example com_config in Joomla core is already using the new MVC classes.
However as you probably know the
Jxxx classes are pretty much empty so a lot of the work will be putting in the same level of code as in the current
JxxxLegacy classes. This will mean that when you write a component you should need to write as little code as possible.
That's where I come in - I'm going to be writing a full set of
Jxxx classes for (probably) J3.5 probably as part of Joomla's GSoC campaign this year (2014). This will also involve swapping a lot of the components over to use those new (hopefully weblinks, newfeeds and contacts for an example for 3PD's as well as some of the more internally used components such as cache.)
However note these classes will not be available in J2.5 which means if you are building a J2.5/3.x compatible extension then you will NOT be able to use them. In terms of removal - I suspect we will remove the Legacy classes in J4.0 however this is not a given - we may even choose to keep them until J5 in order to give extension developers plenty of time to make the transition
The legacy classes aren't deprecated yet at the current time (with Joomla 3.3). So it's perfectly fine to use them for a Joomla 3 extension.
The earliest we will deprecate them is if there are replacement classes in the new MVC. It may be that the GSoC project from George Wilson will provide us with that. But as long as core extensions are using those classes, it doesn't make sense to deprecate them.
The earliest version those can be removed (if deprecated before) will be Joomla 4.0. So for Joomla 3.x you're safe anyway with using those.