Answer:
1.How long have I known Bob
2.How long have Sue and Alan been married?
3.How long has George been unemployed?
4.How long have those books been here?
5.How long has Mary been at the airport?
6.How long has my sister been ill?
7.How long has she had a bad cold?
8.How long is Jan's hair?
9.How long have I had a yacht?
10.How long have they been in the restaurant?
11. How long have my parents been in London?
12.How long has the cat been under the table.
Explanation:
The above questions were grammatically correct by conjugating the verbs according to the pronouns and establishing a nominal and verbal agreement between the words, in order to keep the questions coherent and cohesive.
