How can I count the number of consecutive TRUEs in a DataFrame?

Solution should be simplify, if always at least one True per column:

b = df.cumsum()
c = b.sub(b.mask(df).ffill().fillna(0)).astype(int)

print (c)
   A  B  C
0  0  1  0
1  0  0  0
2  1  1  0
3  2  2  1
4  0  3  0
5  1  4  1
6  2  0  0
7  3  0  1
8  0  1  2
9  1  0  0

#get maximal value of all columns
length = c.max().tolist()
print (length)
[3, 4, 2]

#get indexes by maximal value, subtract length and add 1 
index = c.idxmax().sub(length).add(1).tolist()
print (index)
[5, 2, 7]

Detail:

print (pd.concat([b,
                  b.mask(df), 
                  b.mask(df).ffill(), 
                  b.mask(df).ffill().fillna(0),
                  b.sub(b.mask(df).ffill().fillna(0)).astype(int)
                  ], axis=1, 
                  keys=('cumsum', 'mask', 'ffill', 'fillna','sub')))

  cumsum       mask           ffill           fillna           sub      
       A  B  C    A    B    C     A    B    C      A    B    C   A  B  C
0      0  1  0  0.0  NaN  0.0   0.0  NaN  0.0    0.0  0.0  0.0   0  1  0
1      0  1  0  0.0  1.0  0.0   0.0  1.0  0.0    0.0  1.0  0.0   0  0  0
2      1  2  0  NaN  NaN  0.0   0.0  1.0  0.0    0.0  1.0  0.0   1  1  0
3      2  3  1  NaN  NaN  NaN   0.0  1.0  0.0    0.0  1.0  0.0   2  2  1
4      2  4  1  2.0  NaN  1.0   2.0  1.0  1.0    2.0  1.0  1.0   0  3  0
5      3  5  2  NaN  NaN  NaN   2.0  1.0  1.0    2.0  1.0  1.0   1  4  1
6      4  5  2  NaN  5.0  2.0   2.0  5.0  2.0    2.0  5.0  2.0   2  0  0
7      5  5  3  NaN  5.0  NaN   2.0  5.0  2.0    2.0  5.0  2.0   3  0  1
8      5  6  4  5.0  NaN  NaN   5.0  5.0  2.0    5.0  5.0  2.0   0  1  2
9      6  6  4  NaN  6.0  4.0   5.0  6.0  4.0    5.0  6.0  4.0   1  0  0

EDIT:

General solution working with only False columns – add numpy.where with boolean mask created by DataFrame.any:

print (df)
       A      B      C
0  False   True  False
1  False  False  False
2   True   True  False
3   True   True  False
4  False   True  False
5   True   True  False
6   True  False  False
7   True  False  False
8  False   True  False
9   True  False  False

b = df.cumsum()
c = b.sub(b.mask(df).ffill().fillna(0)).astype(int)

mask = df.any()
length = np.where(mask, c.max(), -1).tolist()
print (length)
[3, 4, -1]

index =  np.where(mask, c.idxmax().sub(c.max()).add(1), 0).tolist()
print (index)
[5, 2, 0]