There's nothing wrong with using a timer, per se. Just as there's nothing fundamentally wrong with any of the thousand and one mindfulness apps, books and gizmos now cluttering the market.
However, unless any of this stuff is used in conjunction with a regular meditation practice, I doubt very much that it will make any meaningful difference to one's life.
Mindfulness isn't something that we "get", like, say, knowledge, and then put it to one side, simply calling on when we need it. Knowledge has to be remembered. The way we remember how to be mindful is to practice it. Try thinking of the meditation as the practice and the rest of your day as the meditation.
If I go a couple of days without practice (like recently when I was ill with shingles - for a few days I resembled The Elephant Man which was kinda interesting), the difference is obvious to me. Thoughts become stickier. Worries tend to linger. I ruminate. I feel much more remote from my own body sensations. When I'm practicing daily, which is most of the time, my life tends to be defined by something like quiet spaciousness - even quiet joy. Within that space I not only feel at ease with myself, I also find the room to manoeuvre so that I can respond to events unfolding rather than react to them.
We can't simply flick a switch in order to be wise. Nor can we simply set a timer and expect to shift into mindful mode. It doesn't work like that. An intellectual understanding of how mindfulness works is never enough. Mindfulness is an experiential practice. It needs to be practiced.
Peter says that doing less than half an hour is like doing nothing. This raises the important question of how much meditation we need to do for it to be in any way meaningful. My argument would be that, while ten minutes a day is much, much better than doing nothing, it's only when we stretch our meditation out that we properly come up against the resistances of boredom, irritation, physical discomfort etc. In five minutes, there's not going to be much opportunity to explore the edges of that resistance which is a vital part of the learning
Personally, I tend to vary things. One day I might do a 40 minute body scan, the next day a 15 minute sit, the next day a 30 minute sit...Sometimes I'll meditate to music (not Motorhead, though I have tried
). If I'm in the park with Banjo I'll do a short breathing space while he larks about, doing what dogs do. For me, it's vital to keep that practice fresh and so I try to vary it as much as I can. I'm also a voracious reader and a big part of my library is reserved for books on mindfulness, meditation, Zen etc. I find that it all feeds in to my practice. In a sense it's all practice. Or, to put it another way, it's all meditation.
Good things,
Jon