Go to matrix.itasoftware.com and search for your flight there. When you find the dates/flights/price that matches what you see on ba.com, click on the link for the full fare rules. This will include the "maximum stay" rule. The precise wording of this rule can be important. Don't just look at the time period that it says as you need to understand exactly what "maximum stay" means.

It's not clear whether you're currently planning to stop at LHR either on the way out or the way back, or whether you're currently intending simply to connect in both directions. If you aren't, but you may want to inject a stop at LHR if you change your plans, you will also want to read the "stopover" rule. You may find that the fare you're currently looking at doesn't allow stopovers, and you'd have to buy a more expensive fare that permits a stopover. That doesn't necessarily mean that you have to buy the more expensive fare now, but if you are allowed to change your booking later to include a stopover, it won't come as an unwelcome surprise that there will be a fare difference payable to allow you to do so because you need to be re-fared to a fare that permits stopovers.