create table users (
    user_id    varchar(10) primary key,
    ... --standard junk you already have here
    turns_left int default 0,
    last_turn  datetime default getdate()
)

create table misc (
    tag varchar(32) primary key,
    val varchar(128)
)
go
insert misc (tag, val) values ('Turns_Per_Day', '5')
insert misc (tag, val) values ('Max_Turns_Allowed', '15')

##</code><code>##

create proc compute_turns_left
    @user_id varchar(10)
as
    declare @days integer, @turns, @turns_per_day, @max_turns

    -- Read the configuration variables
    select @turns_per_day = isnull(convert(int, val), 1)
        from misc where tag='Turns_Per_Day'
    select @max_turns = isnull(convert(int, val), 1)
        from misc where tag='Max_Turns_Allowed'

    -- How many days since they last used a turn?
    select @days=datediff(d,getdate(),last_turn)
        from users where user_id=@user_id

    -- Update their turns
    update turns_left = turns_left + @days * @turns_per_day
    from users where user_id=@user_id

    -- And apply the limit cap
    update turns_left = @max_turns
        from users where user_id=@user_id and turns_left > @max_turns
go

##</code><code>##

create proc consume_turn
    @user_id varchar(10)
as
    update users set turns_left = turns_left-1, last_turn=getdate()
        where user_id=@user_id
go