The studies I've seen on rebalancing ignore risk, so they conclude that waiting is better. Most often stocks do better than bonds, so the more stocks in your portfolio, the better it performs (relative to holding bonds). So if you target 60% stocks / 40% bonds, letting it drift to 75% stocks gives better performance - but it's very far from the original allocation. I haven't seen studies consider risk or acceptable ranges when they pick a rebalancing interval. So with those caveats, the studies I read suggested annual rebalancing.
You might look into "rebalancing bands". A portfolio targetting 60% stocks might be allowed to drift in the range of 55% - 65%, and then rebalance at 54% or 66% stocks. A spreadsheet makes it easier - you just glance and see the current percentage.
Thanks for this information. I will have to look into this more and update my spreadsheet with some more granularity to allow for this once I delve into bonds close to retirement.
