Hello High Profile
There is internal guidance about this from the IDIs
Visitors
4. FREQUENCY AND DURATION OF VISITS
There is no restriction on the number of visits a person may make to the United Kingdom nor any requirement that a specified time must elapse between successive visits. The fact that a person has made a series of visits with only brief intervals between them would not, in the absence of any other relevant factors, constitute sufficient ground for refusal. It is reasonable, however, for the immigration officer to consider the stated purpose of the visit in the light of the length of time that has elapsed since previous visits. A visitor should not, for example, normally spend more than 6 out of any 12 months in this country (but see Section 3 of this chapter in the case of visitors for private medical treatment). Occasionally, a business visitor may require to stay for a period of weeks or even months (eg where machinery is being installed or faults being diagnosed and corrected). The immigration officer should be satisfied, however, that a person's presence here on business for more than 6 out of any 12 months does not mean that he is basing himself here ..........
More than 6 months out of 12 is running the gauntlet a bit- it seems your mother would be in the UK for 10 months out of 12 (you haven't given exact dates).
How important is it that your mother is here? What is she doing here? Has she got a return ticket? What about funds? What would happen if your mother is refused entry? - all things to consider.
PP
