Grunt and estrous calls only work when the deer are in the right mood. Pre-rut and the rut are probably the best times, although I have called in bucks during early bow season with a young buck grunt. I've also called back deer that I had missed with my bow and got a second shot. I think for deer calls to be effective on a regular bases, you need a balanced deer herd where there is competition for the rights to mate the does. Don't really have to worry about that up here in VT. As far as what time of the day to call, I don't think that matters much but if you have a deer close buy and you've already called to it and it's coming in, I wouldn't call again. If it appears to be leaving, then it won't hurt but don't call to loud. I made that mistake as a young hunter and it didn't take the deer long to leave the area.