Well the game servers have been online and running since at least late February and the cars that reviewers/early players were seeing back then (Feb 24th) haven't turned up again yet (March 19th) and aren't likely to any time soon when there is probably still another 20 or 30 cars still to come up for the first time.
So in short, I think we're looking at two months, maybe more, for a car to return to LCD at the current rate.
EDIT: Since Feb 24 (The earliest we have any reliable data) these are the 20 cars cars that have appeared:
Alfa Romeo | 155 2.5 V6 TI |
Alfa Romeo | Giulia TZ2 Carrozzata da Zagato CN.AR750106 |
Aston Martin | DB5 |
Audi | Sport quattro S1 |
Chevrolet | Corvette (C2) Sting Ray Sport Coupe |
Ferrari | 250 GT Berlinetta passo corto CN.2521 |
Ferrari | 500 Mondial Pinin Farina Coupe |
Ferrari | F40 |
Ferrari | F50 |
Ford | Mark IV Race Car |
Jaguar | E-Type Coupe |
Jaguar | XJ13 |
Mercedes AMG | Sauber C9 |
Nissan | Fairlady Z (S30) Z432 |
Plymouth | XNR Ghia Roadster |
Porsche | 356 A/1500 GS Carrera |
Porsche | 911 996 GT1 Strassenversion |
Shelby | Cobra Daytona Coupe |
Shelby | GT350 |
Toyota | Supra GT500 |
There are 62 cars we've not seen yet, and my guess is that 37 of them will be legendary cars. It may be more, but these seem solid calls to me
Abarth | 1500 Biposto Bertone B.A.T 1 | 1952 |
Alfa Romeo | 8C 2900B Lungo Berlinetta | 1937 |
Alpine | A220 | 1968 |
Aston Martin | DB3S CN.1 | 1953 |
BMW | 3.0 CSL | 1972 |
BMW | McLaren F1 GTR Race Car | 1997 |
Chaparral | Chaparral 2J | 1970 |
Chevrolet | Camaro Z28 | 1969 |
Chevrolet | Corvette (C3) Stingray L46 350 | 1969 |
Chevrolet | Corvette Stingray Concept | 1959 |
De Tomaso | Mangusta | 1969 |
Ferrari | 250 GTO CN.3729GT | 1962 |
Ferrari | 308 GTB 1976 | 1976 |
Ferrari | 330 P4 | 1967 |
Ferrari | 365 GTB4 | 1971 |
Ford | GT40 Mk1 | 1966 |
Ford | Mustang Boss 429 | 1969 |
Jaguar | D-Type | 1954 |
Jaguar | XJR-9 | 1988 |
Lamborghini | Miura P400 Bertone Prototype CN.0706 | 1967 |
Lancia | Delta HF Integrale Evoluzione | 1991 |
Lancia | Stratos | 1973 |
Mazda | 787B | 1991 |
McLaren | F1 | 1994 |
McLaren | F1 GTR Kokusai Kaihatsu | 1995 |
Mercedes-Benz | 300 SEL 6.8 AMG | 1971 |
Mercedes-Benz | 300SL W194 | 1952 |
Mercedes-Benz | 300 SL Coupe | 1954 |
Nissan | R92CP | 1992 |
Nissan | Skyline KPGC10 GT-R | 1971 |
Nissan | Skyline R34 GT-R GT500 | 1999 |
Peugeot | 205 T16 Evolution II Rally Car | 1986 |
Porsche | 356 A/1500 GS GT Carrera Speedster | 1956 |
Porsche | 962 C | 1988 |
Porsche | Spyder Type 550/1500 RS | 1955 |
Renault | R8 Gordini | 1966 |
Shelby | Cobra 427 | 1966 |
Plus there are the four cars we only saw in pre-release footage. So a total of 61 legendary cars by my estimates. If they stick to the current rate of just under one new a day (20 in 23 days so far) then my estimate would be correct, a car will appear roughly once every 66 days.