Typically 125 years. Some are offered at only 99 years but that is becoming more unusual
You will not own land, you will simply have a right to occupy part of someone else’s property for a term of years and at the end of that term of years you must leave and have no further rights. You will also be obliged to pay a ground rent each year to the freeholder. This is usually a payment of £100-£250 per year. You will also have to contribute to the costs of maintaining and insuring the building, sometimes known as a service charge, these charges can be quite expensive in the £1,000-£2500 range depending on the location and type of building.
The premises will revert to the landlord and you no longer have a right to stay there.
Yes, they are not always mortgageable unless you own the freehold as well or a share in the freehold.
Unless the lease specifically prohibits it (most don’t) then you can. Ask your solicitor to check the lease to see if there are any prohibitions or conditions related to sub-letting.